Explore every pathology specialization with curated articles, clinical cases, diagnostic guides, leading professionals, jobs and educational resources.
The cornerstone of diagnostic medicine. Anatomical pathology encompasses the examination of tissues and organs to diagnose disease, guide treatment, and determine prognosis through gross, microscopic, and molecular evaluation.
Surgical pathology is the most widely practised branch of anatomical pathology, involving the gross and microscopic examination of surgically excised specimens. It encompasses grossing protocols for specimen orientation and margin assessment, intraoperative frozen-section consultation for rapid diagnosis during surgery, and definitive tumour staging using systems such as TNM and FIGO. Surgical pathologists also evaluate margin status, lymphovascular invasion, and perineural involvement, all of which directly influence clinical management and prognosis.
Cytopathology focuses on cellular-level diagnosis obtained through fine-needle aspiration (FNA), exfoliative preparations, and body fluid analysis. The discipline includes cervical screening via conventional Pap smears and liquid-based cytology (LBC) preparations such as ThinPrep and SurePath, as well as FNA of thyroid, lymph nodes, salivary glands, and deep-seated lesions. Modern cytopathology integrates ancillary testing including immunocytochemistry, molecular analysis, and cell block preparations to enhance diagnostic accuracy and guide targeted therapy decisions.
Dermatopathology bridges dermatology and pathology, focusing on the microscopic interpretation of skin biopsies to diagnose inflammatory, infectious, and neoplastic cutaneous conditions. Melanocytic lesion assessment, including the distinction between benign naevi and melanoma using criteria such as asymmetry, maturation, and pagetoid spread, remains one of the most challenging areas in diagnostic pathology. Inflammatory dermatopathology requires pattern-based analysis of spongiotic, lichenoid, psoriasiform, and vasculitic reactions. The subspecialty also covers Mohs micrographic surgery margin evaluation, direct immunofluorescence for autoimmune blistering diseases, and sentinel lymph node assessment for cutaneous malignancies.
Neuropathology encompasses the diagnosis of diseases affecting the central and peripheral nervous systems, skeletal muscle, and the eye. The WHO Classification of Tumours of the Central Nervous System integrates histological features with molecular markers such as IDH mutation, 1p/19q codeletion, MGMT promoter methylation, and histone H3 alterations for precise glioma subtyping. Neurodegenerative disease diagnosis relies on the identification of specific proteinopathies including tau, alpha-synuclein, TDP-43, and amyloid-beta, with staging systems such as Braak for Alzheimer disease and Parkinson disease. The subspecialty also covers demyelinating diseases, CNS vasculitis, infectious encephalitis, and nerve and muscle biopsy interpretation for peripheral neuropathies and myopathies.
Gastrointestinal pathology covers the diagnostic evaluation of biopsy and resection specimens from the oesophagus, stomach, small bowel, colon, liver, gallbladder, and pancreas. Key areas include inflammatory bowel disease (IBD) assessment with distinction between Crohn disease and ulcerative colitis based on distribution patterns and granuloma presence, Barrett oesophagus surveillance with dysplasia grading, and coeliac disease diagnosis using Marsh-Oberhuber classification. GI oncology encompasses adenocarcinoma staging, neuroendocrine tumour grading using Ki-67 index, gastrointestinal stromal tumour (GIST) risk stratification, and molecular classification of colorectal carcinoma including microsatellite instability testing, KRAS/NRAS/BRAF mutation analysis, and mismatch repair protein immunohistochemistry.
Breast pathology is a high-volume subspecialty central to multidisciplinary breast cancer management. Core needle biopsy interpretation follows the B-classification system (B1-B5) for radiological correlation, while excision specimens require meticulous margin assessment and tumour staging. Biomarker evaluation, including oestrogen receptor (ER), progesterone receptor (PR), HER2 status by immunohistochemistry and in situ hybridisation, and Ki-67 proliferation index, is essential for treatment stratification. The Nottingham grading system provides histological grading, while molecular classification into luminal A, luminal B, HER2-enriched, and basal-like subtypes guides systemic therapy selection. Sentinel lymph node evaluation, DCIS grading, and assessment of treatment response using the residual cancer burden (RCB) system are integral to the subspecialty.
Uropathology encompasses the diagnosis of neoplastic and non-neoplastic diseases of the kidney, ureter, bladder, prostate, testis, and penis. Prostate cancer diagnosis centres on the Gleason grading system, now reported using the ISUP Grade Group classification (Grades 1-5), with assessment of perineural invasion, extraprostatic extension, and surgical margin status on radical prostatectomy specimens. Bladder cancer staging requires evaluation of lamina propria and muscularis propria invasion, with the WHO/ISUP classification distinguishing low-grade from high-grade urothelial neoplasms. Renal tumour pathology has been transformed by the integration of molecular markers, with the WHO classification recognising entities defined by specific genetic alterations such as TFE3 translocation renal cell carcinoma. The subspecialty also covers medical renal pathology including glomerulonephritis, with immunofluorescence and electron microscopy essential for complete assessment.
Gynaecological pathology covers neoplastic and non-neoplastic conditions of the cervix, uterus, fallopian tubes, ovaries, and vulva. Cervical pathology has been revolutionised by the understanding of HPV-driven carcinogenesis, with p16 immunohistochemistry and HPV testing integrated into screening protocols and the LAST Project recommendations guiding squamous lesion terminology. Endometrial pathology encompasses hyperplasia classification using the WHO and EIN systems, endometrial carcinoma molecular classification (POLE-ultramutated, MSI-high, copy-number low, copy-number high) as established by TCGA, and assessment of myometrial invasion depth. Ovarian tumour classification spans surface epithelial, sex cord-stromal, and germ cell categories, with high-grade serous carcinoma representing the most common and lethal subtype, now understood to frequently originate in the fallopian tube fimbria as serous tubal intraepithelial carcinoma (STIC).
Haematopathology integrates morphological, immunophenotypic, cytogenetic, and molecular data for the diagnosis and classification of haematological malignancies and reactive conditions. The WHO Classification of Haematolymphoid Tumours and the parallel International Consensus Classification (ICC) define disease entities by genetic drivers, including recurrent translocations in acute leukaemia, JAK2/CALR/MPL mutations in myeloproliferative neoplasms, and specific immunoglobulin gene rearrangements in lymphoma subtypes. Bone marrow biopsy interpretation encompasses assessment of cellularity, lineage maturation, fibrosis grading, and blast enumeration. Flow cytometry immunophenotyping is indispensable for lineage assignment and minimal residual disease monitoring. Lymph node pathology requires recognition of architectural patterns, follicular dendritic cell networks, and the distinction between reactive hyperplasia and lymphoma.
Pulmonary pathology addresses neoplastic, inflammatory, and fibrotic diseases of the lung and pleura. Lung carcinoma classification requires histological subtyping of adenocarcinoma using the IASLC/ATS/ERS system with assessment of predominant pattern (lepidic, acinar, papillary, micropapillary, solid), predictive biomarker testing for EGFR, ALK, ROS1, PD-L1, KRAS G12C, and other actionable targets, and accurate staging of resection specimens. Interstitial lung disease evaluation involves multidisciplinary diagnosis integrating clinical, radiological, and pathological findings, with surgical lung biopsy patterns including usual interstitial pneumonia (UIP), nonspecific interstitial pneumonia (NSIP), organising pneumonia, and diffuse alveolar damage. Small biopsy and cytology specimens present unique challenges requiring judicious tissue stewardship for molecular testing.
Head and neck pathology covers a diverse array of neoplastic and non-neoplastic conditions of the oral cavity, oropharynx, larynx, sinonasal tract, salivary glands, and thyroid. HPV-associated oropharyngeal squamous cell carcinoma is now recognised as a distinct entity with favourable prognosis, diagnosed by p16 immunohistochemistry and confirmed by HPV ISH or PCR. Salivary gland tumour classification encompasses a wide morphological spectrum from pleomorphic adenoma to adenoid cystic carcinoma, with novel entities defined by specific gene fusions such as secretory carcinoma (ETV6-NTRK3). Thyroid pathology includes the Bethesda System for reporting thyroid cytopathology, risk stratification of follicular-patterned lesions (NIFTP), and molecular testing panels to guide management of indeterminate nodules.
Bone and soft tissue pathology is a subspecialty demanding expertise in the diagnosis of a wide variety of mesenchymal tumours, many of which are rare and characterised by specific molecular alterations. Sarcoma diagnosis requires integration of morphological pattern recognition, immunohistochemistry, and molecular testing for characteristic translocations such as SS18-SSX in synovial sarcoma, EWSR1-FLI1 in Ewing sarcoma, and MDM2 amplification in well-differentiated and dedifferentiated liposarcoma. The FNCLCC grading system stratifies soft tissue sarcomas by differentiation, mitotic count, and necrosis. Non-neoplastic conditions include metabolic bone disease, Paget disease, avascular necrosis, and crystal arthropathies requiring polarised light microscopy. Bone tumour assessment integrates radiological correlation for appropriate classification and staging.
Clinical pathology encompasses the laboratory disciplines that analyse blood, urine, and other body fluids to provide diagnostic, prognostic, and therapeutic monitoring information for patient care.
Chemical pathology, also known as clinical biochemistry, focuses on the biochemical analysis of body fluids for disease diagnosis, monitoring, and screening. The discipline encompasses liver function tests (bilirubin, transaminases, ALP, GGT, albumin), renal function assessment (creatinine, eGFR, cystatin C, electrolytes, acid-base analysis), endocrine investigations (thyroid function, cortisol dynamics, HbA1c, insulin, growth hormone, reproductive hormones), lipid profiles, cardiac biomarkers (troponin, BNP/NT-proBNP), tumour markers (PSA, CA-125, AFP, CEA, CA 19-9), and therapeutic drug monitoring. Point-of-care testing, laboratory automation, reference range validation, and method comparison studies are critical operational components. The field increasingly incorporates metabolomics and proteomics for biomarker discovery.
Clinical haematology spans diagnostic morphology, coagulation science, and laboratory management of blood disorders. Peripheral blood film examination remains a cornerstone skill, enabling identification of blast cells, dysplastic features, red cell morphology abnormalities (spherocytes, schistocytes, target cells, sickle cells), and parasites including malaria species identification. Coagulation assessment encompasses PT/INR, APTT, fibrinogen, D-dimer, mixing studies, and specialised factor assays for inherited and acquired bleeding disorders, thrombophilia screening (protein C, protein S, antithrombin, lupus anticoagulant, antiphospholipid antibodies), and heparin-induced thrombocytopenia workup. Full blood count interpretation, reticulocyte analysis, haemoglobinopathy screening (HPLC, capillary electrophoresis), and iron studies complete the diagnostic armamentarium.
Medical microbiology provides identification and susceptibility testing of pathogenic micro-organisms to guide antimicrobial therapy and infection control. Bacteriology encompasses culture-based methods, automated identification systems (MALDI-TOF mass spectrometry), and antimicrobial susceptibility testing following EUCAST and CLSI breakpoints, with attention to multidrug-resistant organisms including MRSA, ESBL-producing Enterobacterales, carbapenemase producers, and vancomycin-resistant enterococci. Virology has been transformed by molecular diagnostics, with multiplex respiratory panels, quantitative viral load monitoring (HIV, HBV, HCV, CMV, EBV), and sequencing for resistance mutations now standard practice. Mycology covers dermatophyte identification, invasive aspergillosis biomarkers (galactomannan, beta-D-glucan), and Candida species identification. Parasitology, mycobacteriology, antimicrobial stewardship, and infection prevention round out this expansive discipline.
Clinical immunology encompasses the laboratory investigation of immune-mediated diseases, immunodeficiency, and allergy. Autoimmune serology includes antinuclear antibody (ANA) testing by indirect immunofluorescence with pattern recognition (homogeneous, speckled, centromere, nucleolar), anti-dsDNA, ENA panel (Sm, RNP, SSA/Ro, SSB/La, Scl-70, Jo-1), ANCA (cANCA/PR3, pANCA/MPO), and disease-specific antibodies such as anti-CCP for rheumatoid arthritis and anti-tissue transglutaminase for coeliac disease. Immunoglobulin quantification, serum protein electrophoresis with immunofixation for paraprotein detection, complement studies (C3, C4, CH50), and lymphocyte subset enumeration by flow cytometry support diagnosis and monitoring. Allergy testing includes specific IgE measurement, basophil activation testing, and component-resolved diagnostics.
Transfusion medicine ensures the safe and effective use of blood products through donor screening, component preparation, compatibility testing, and adverse reaction management. ABO and RhD blood group serology, antibody screening and identification using panel cells, direct and indirect antiglobulin testing (DAT/IAT), and crossmatching procedures form the foundation. The discipline manages complex clinical scenarios including autoimmune haemolytic anaemia with warm or cold autoantibodies, haemolytic disease of the fetus and newborn (HDFN), massive transfusion protocols, and platelet refractoriness investigation. Patient blood management programmes promote evidence-based transfusion practice, restrictive transfusion thresholds, and alternatives to allogeneic transfusion. Apheresis services provide therapeutic plasma exchange, leucapheresis, and stem cell collection for transplantation.
Forensic pathology applies pathological principles to medicolegal death investigation, determining cause, mechanism, and manner of death. The subspecialty encompasses external examination and autopsy technique, wound interpretation (blunt force, sharp force, firearm injuries), asphyxia classification (ligature, manual, positional, drowning), toxicology correlation for drug-related deaths, and scene interpretation. Death certification, coronial and medico-legal reporting, and expert witness testimony are integral to practice. Paediatric forensic pathology addresses sudden unexpected death in infancy (SUDI), non-accidental injury recognition, and the distinction between inflicted and accidental injury. The discipline intersects with forensic anthropology, forensic entomology, and molecular identification in cases of decomposed or skeletonised remains.
Technology-driven disciplines reshaping diagnostic pathology, from whole-slide imaging and artificial intelligence to next-generation sequencing and spatial multi-omics, defining the future of precision diagnostics.
Digital pathology involves the acquisition, management, and interpretation of pathology information in a digital environment, centred on whole-slide imaging (WSI) technology. Modern scanners digitise glass slides at 40x magnification producing high-resolution images suitable for primary diagnosis, now approved for clinical use by the FDA and CE-marked in Europe. Implementation requires robust IT infrastructure, validated image management systems, standardised colour calibration, and quality assurance protocols. Digital pathology enables remote consultation, telepathology services, multidisciplinary team review, standardised education, and quantitative image analysis. Integration with laboratory information systems (LIS) and electronic health records (EHR) supports workflow efficiency, while DICOM standards and IHE Pathology profiles facilitate interoperability.
Computational pathology applies machine learning, deep learning, and computer vision to histopathological image analysis for automated diagnosis, grading, and prognosis prediction. Convolutional neural networks (CNNs) and vision transformers trained on large annotated datasets can detect and classify tumours, quantify biomarker expression (ER, PR, HER2, Ki-67, PD-L1), enumerate mitoses, and predict molecular alterations directly from H&E-stained slides. Foundation models pre-trained on millions of pathology images offer transfer learning capabilities across multiple tasks. Explainability methods, including attention maps and gradient-based visualisation, address the interpretability challenge. Regulatory pathways (FDA De Novo, 510(k), CE-IVDR) govern the approval and post-market surveillance of AI-based diagnostic devices. Prospective clinical validation, bias assessment, and integration into pathologist workflows remain active areas of research.
Molecular diagnostics has become indispensable for precision oncology, providing actionable genomic information that guides targeted therapy and immunotherapy selection. Next-generation sequencing (NGS) panels interrogate hundreds of cancer-relevant genes simultaneously for single-nucleotide variants, insertions/deletions, copy number alterations, and gene fusions from formalin-fixed paraffin-embedded (FFPE) tissue. Liquid biopsy through circulating tumour DNA (ctDNA) analysis enables non-invasive genotyping, treatment response monitoring, minimal residual disease detection, and early identification of resistance mutations. Companion diagnostic assays pair specific biomarker tests with approved therapies, such as EGFR mutations with osimertinib, ALK fusions with crizotinib/alectinib, and BRAF V600E with dabrafenib/trametinib. RNA-based fusion panels, tumour mutational burden (TMB) assessment, and microsatellite instability (MSI) testing complete the molecular diagnostic toolkit.
Pharmacogenomics studies how germline genetic variation influences drug metabolism, efficacy, and adverse reactions, enabling individualised prescribing. Key pharmacogenes include CYP2D6 and CYP2C19 (affecting metabolism of antidepressants, antipsychotics, opioids, and clopidogrel), DPYD (fluoropyrimidine toxicity risk), TPMT and NUDT15 (thiopurine metabolism), UGT1A1 (irinotecan toxicity), and HLA-B*57:01 (abacavir hypersensitivity). The Clinical Pharmacogenetics Implementation Consortium (CPIC) and Dutch Pharmacogenetics Working Group (DPWG) provide evidence-based dosing guidelines translated into clinical decision support within electronic health records. Pre-emptive pharmacogenomic testing panels enable prospective genotyping of multiple actionable variants from a single sample, with star allele assignment and metaboliser phenotype prediction facilitating dose adjustments before drug initiation.
Spatial transcriptomics represents a paradigm shift in pathology by enabling gene expression profiling while preserving tissue architecture and spatial context. Technologies such as Visium (10x Genomics), MERFISH, seqFISH, and Slide-seq map thousands of transcripts to precise tissue locations, revealing cell-type heterogeneity, tumour microenvironment organisation, and spatially resolved signalling pathways. Integration with spatial proteomics platforms, including multiplexed ion beam imaging (MIBI), imaging mass cytometry (IMC), and CODEX, provides simultaneous protein-level characterisation of dozens to hundreds of markers on a single tissue section. Single-cell RNA sequencing deconvolution methods complement spatial data by providing high-resolution cell-type annotations. These approaches are transforming understanding of tumour-immune interactions, metastatic niches, and tissue development, with translational applications in biomarker discovery and therapeutic target identification.
Laboratory informatics encompasses the information systems and data management strategies that underpin modern pathology services. Laboratory information systems (LIS) and laboratory information management systems (LIMS) manage specimen tracking, result reporting, and quality control workflows. Middleware solutions integrate analyser interfaces, auto-verification rules, and delta-check algorithms to enhance efficiency and reduce turnaround times. Natural language processing applied to pathology reports enables structured data extraction from free-text narratives for cancer registry, research databases, and quality metrics. Interoperability standards including HL7 FHIR and SNOMED CT facilitate data exchange across healthcare systems. Data governance, cybersecurity, and compliance with regulations such as GDPR and HIPAA are critical for protecting sensitive patient information in the digital pathology era.
Cross-cutting disease categories that span multiple organ systems and subspecialties, integrating morphological, molecular, and clinical perspectives for comprehensive diagnostic understanding.
Cancer pathology is the diagnostic foundation of oncology, providing histological diagnosis, grading, staging, and molecular characterisation that determine treatment selection and predict clinical outcome. The AJCC/UICC TNM staging system provides standardised assessment of tumour size (T), regional lymph node involvement (N), and distant metastasis (M) across all solid tumour types, with each edition incorporating evidence-based prognostic refinements. Histological grading systems are tumour-type specific: Nottingham for breast, Gleason/ISUP for prostate, FNCLCC for soft tissue sarcoma, and Fuhrman/WHO-ISUP for renal cell carcinoma. Molecular classification increasingly supplements morphological diagnosis, with examples including PAM50 intrinsic subtypes in breast cancer, TCGA molecular groups in endometrial carcinoma, consensus molecular subtypes (CMS) in colorectal cancer, and integrated molecular classification of gliomas. Predictive biomarker testing for targeted therapies and immunotherapies, including PD-L1 tumour proportion score, mismatch repair status, and comprehensive genomic profiling, is now standard of care across multiple tumour types.
Inflammatory disease pathology spans virtually every organ system, requiring pattern recognition across tissue types to distinguish acute, chronic, granulomatous, and autoimmune inflammatory processes. Autoimmune conditions demonstrate characteristic histopathological features: lupus nephritis is classified by the ISN/RPS system into six classes based on glomerular involvement pattern, autoimmune hepatitis shows interface hepatitis with plasma cell infiltrates, and autoimmune blistering diseases are defined by the level of blister formation and direct immunofluorescence findings. Granulomatous inflammation demands a differential diagnosis including sarcoidosis, mycobacterial and fungal infections, Crohn disease, foreign body reaction, and vasculitis. IgG4-related disease is a recently recognised fibro-inflammatory condition with characteristic storiform fibrosis, dense lymphoplasmacytic infiltrate, obliterative phlebitis, and elevated IgG4-positive plasma cells. Vasculitis classification follows the Chapel Hill Consensus nomenclature by vessel size.
Infectious disease pathology integrates histomorphological pattern recognition with special stains, immunohistochemistry, and molecular testing to identify causative organisms in tissue. Tissue reaction patterns provide diagnostic clues: necrotising granulomas suggest mycobacterial or fungal infection, suppurative granulomas indicate cat-scratch disease or lymphogranuloma venereum, and viral cytopathic changes (Cowdry type A inclusions, ground-glass hepatocytes, koilocytosis) point to specific viral aetiologies. Special stains remain valuable, including Grocott methenamine silver (GMS) for fungi, Ziehl-Neelsen for acid-fast bacilli, and Warthin-Starry for spirochaetes and Bartonella. Immunohistochemistry for CMV, HSV, adenovirus, and Toxoplasma provides specific identification. Molecular techniques including PCR, in situ hybridisation (ISH), and metagenomic sequencing increasingly complement morphology, particularly for fastidious organisms and in immunocompromised patients where tissue responses are atypical.
Metabolic disease pathology covers the tissue manifestations of inherited and acquired metabolic disorders, storage diseases, and nutritional deficiencies. Liver pathology frequently reflects systemic metabolic derangement: non-alcoholic fatty liver disease (NAFLD) and its progressive form, metabolic dysfunction-associated steatohepatitis (MASH), are assessed using the NAS score and fibrosis staging. Wilson disease shows characteristic copper accumulation with rhodanine and orcein staining, while hereditary haemochromatosis demonstrates iron deposition quantifiable by Perls Prussian blue stain and hepatic iron concentration. Lysosomal storage diseases, including Gaucher, Fabry, and Niemann-Pick disease, demonstrate pathognomonic cellular changes identifiable by light microscopy and confirmed by electron microscopy, enzyme assays, and genetic testing. Amyloidosis diagnosis requires Congo red staining with apple-green birefringence under polarised light, with mass spectrometry-based typing now considered the gold standard for amyloid subtype identification.
Transplant pathology is dedicated to the histological assessment of allograft biopsies for rejection, infection, drug toxicity, and disease recurrence. Kidney transplant biopsy interpretation follows the Banff classification, which defines diagnostic categories including T-cell-mediated rejection (with tubulitis and intimal arteritis grading), antibody-mediated rejection (characterised by microvascular inflammation, C4d deposition, and donor-specific antibody correlation), and chronic allograft nephropathy. Liver allograft biopsies are evaluated using the Banff schema for rejection assessment, distinguishing acute cellular rejection (portal inflammation, bile duct damage, venous endothelialitis) from chronic ductopenic rejection. Heart transplant endomyocardial biopsy follows the ISHLT grading system for cellular and antibody-mediated rejection. Lung transplant pathology addresses acute cellular rejection, lymphocytic bronchiolitis, and the critical diagnosis of chronic lung allograft dysfunction including bronchiolitis obliterans syndrome.
Paediatric pathology addresses the unique spectrum of diseases affecting fetuses, neonates, infants, and children, including developmental anomalies, inherited conditions, and age-specific neoplasms. Placental pathology provides critical information about pregnancy complications, with examination protocols assessing villous maturation, maternal and fetal vascular malperfusion patterns, chorioamnionitis staging and grading, and chronic inflammatory lesions. Childhood tumours include embryonal neoplasms (Wilms tumour, neuroblastoma, hepatoblastoma, retinoblastoma, rhabdomyosarcoma), which require risk stratification by histological subtype, stage, and molecular markers. The International Neuroblastoma Pathology Classification (INPC) and Children's Oncology Group (COG) protocols guide treatment decisions. Perinatal autopsy demands specialised expertise in fetal and neonatal anatomy, growth assessment, dysmorphology, and the investigation of stillbirth and neonatal death.
The methodological toolkit underpinning modern pathological diagnosis, from classical histochemistry and immunohistochemistry to advanced molecular and proteomic techniques.
Histochemistry employs chemical reactions to demonstrate specific substances within tissue sections. The haematoxylin and eosin (H&E) stain remains the universal foundation. Special stains include PAS for glycogen and basement membranes, Masson trichrome for collagen and fibrosis, reticulin for hepatic architecture, elastic van Gieson for vascular structures, Congo red for amyloid, Perls Prussian blue for iron, and Alcian blue for mucins. Organism-detecting stains such as Grocott-Gomori methenamine silver (GMS), Ziehl-Neelsen (ZN), Wade-Fite, Gram, and Warthin-Starry remain indispensable for infectious disease pathology despite the advent of immunohistochemistry and molecular methods.
Immunohistochemistry (IHC) uses antibodies to detect specific antigens in tissue sections, providing lineage identification, prognostic stratification, and predictive biomarker assessment. Diagnostic panels for tumour classification include cytokeratins for carcinoma subtyping, melanocytic markers (S100, SOX10, Melan-A, HMB45), lymphoid markers (CD20, CD3, CD30, CD15), and neuroendocrine markers (chromogranin, synaptophysin). Predictive biomarkers assessed by IHC include ER, PR, HER2 (with scoring guidelines from ASCO/CAP), PD-L1 (multiple scoring algorithms for different tumour types), and mismatch repair proteins (MLH1, PMS2, MSH2, MSH6). Dual-colour and multiplex IHC enable simultaneous assessment of multiple markers on a single section. Quality assurance encompasses antibody validation, positive and negative controls, and external quality assessment participation.
Transmission electron microscopy (TEM) remains essential for specific diagnostic scenarios where ultrastructural features are pathognomonic. In medical renal pathology, TEM identifies immune complex deposits (subepithelial, subendothelial, mesangial), effacement of podocyte foot processes in minimal change disease and focal segmental glomerulosclerosis, and the fibrillary deposits of amyloidosis and fibrillary glomerulonephritis. In tumour pathology, ultrastructural features such as melanosomes, neurosecretory granules, Birbeck granules, and Weibel-Palade bodies aid lineage determination. Ciliary dyskinesia diagnosis requires examination of dynein arm structure. Proper tissue fixation in glutaraldehyde, processing, thin-sectioning, and heavy metal staining (uranyl acetate, lead citrate) are critical for optimal ultrastructural preservation.
Flow cytometry enables rapid multiparameter analysis of individual cells in suspension, characterising surface and intracellular markers for immunophenotyping, DNA ploidy analysis, and functional assays. In haematopathology, flow cytometry is essential for leukaemia and lymphoma classification, providing lineage assignment (B-cell, T-cell, myeloid), aberrant antigen detection, light chain restriction demonstration in B-cell neoplasms, and blast enumeration. Minimal residual disease (MRD) monitoring uses high-sensitivity panels with 8-10 colour combinations to detect one abnormal cell per 10,000-100,000 normal cells. Lymphocyte subset analysis (CD4/CD8 enumeration for HIV monitoring), paroxysmal nocturnal haemoglobinuria screening (FLAER assay), and stem cell enumeration for transplant harvest adequacy are additional critical applications.
Molecular pathology techniques span nucleic acid extraction, amplification, detection, and sequencing methods applied to diagnostic specimens. PCR-based methods include real-time quantitative PCR (qPCR) for pathogen detection and viral load monitoring, allele-specific PCR for hotspot mutations, and digital PCR for ultra-sensitive detection of rare variants. In situ hybridisation (ISH) methods, including fluorescence ISH (FISH) for gene amplification (HER2), translocation detection (BCR-ABL, ALK, ROS1), and chromosome enumeration, as well as chromogenic ISH (CISH) and RNA ISH for transcript localisation, provide spatial context. Sanger sequencing remains relevant for confirmation of variants and Ig/TCR gene clonality assessment. Methylation analysis, including MGMT promoter methylation in gliomas and MLH1 promoter methylation in colorectal cancer, utilises methylation-specific PCR and pyrosequencing.
Mass spectrometry has become integral to multiple pathology disciplines. MALDI-TOF MS has revolutionised clinical microbiology, providing rapid and accurate bacterial and fungal identification from colonies or directly from positive blood cultures within minutes, replacing conventional biochemical identification methods. In chemical pathology, liquid chromatography-tandem mass spectrometry (LC-MS/MS) provides superior specificity for steroid hormone panels, therapeutic drug monitoring, toxicology screening, and newborn screening. Laser-capture microdissection coupled with mass spectrometry enables amyloid typing from formalin-fixed tissue, now considered the gold standard for amyloid subtype identification. Proteomics applications include biomarker discovery, and matrix-assisted laser desorption/ionisation imaging mass spectrometry (MALDI-IMS) enables spatial mapping of molecules directly on tissue sections.
Cytogenetics provides karyotype analysis and targeted assessment of chromosomal abnormalities critical for haematological malignancy classification, constitutional genetics, and reproductive medicine. Conventional G-banded karyotyping identifies numerical abnormalities (trisomies, monosomies), structural rearrangements (translocations, deletions, inversions), and complex karyotypes. FISH probes target specific loci for rapid detection of diagnostically and prognostically significant abnormalities, including BCR-ABL1 in CML, PML-RARA in APL, and MYC/BCL2/BCL6 in high-grade B-cell lymphoma. Chromosomal microarray analysis (CMA) using SNP arrays detects copy number variants and regions of loss of heterozygosity at higher resolution than conventional karyotyping. Optical genome mapping is an emerging technology providing ultra-long-range structural variant detection complementary to NGS.
Tissue processing transforms fresh clinical specimens into microscopic sections suitable for diagnosis, encompassing fixation, dehydration, clearing, embedding, microtomy, and staining. Formalin fixation and paraffin embedding (FFPE) is the standard method, with 10% neutral buffered formalin providing optimal morphological preservation and compatibility with immunohistochemistry and molecular testing. Cold ischaemia time, fixation duration (6-72 hours depending on specimen type), and pre-analytical variables significantly impact biomarker assessment, particularly HER2 and hormone receptor immunohistochemistry. Frozen section technique enables rapid intraoperative diagnosis within 15-20 minutes using a cryostat. Decalcification methods for bone specimens, tissue microarray construction, and biobanking protocols for fresh-frozen tissue preservation at -80C or in liquid nitrogen support research applications.
The autopsy remains a vital quality assurance tool and the definitive method for determining cause of death, identifying unsuspected disease, and advancing medical knowledge. Hospital (consented) autopsies assess the accuracy of clinical diagnoses, evaluate treatment effects, and identify unexpected pathology, with discrepancy rates between clinical and autopsy diagnoses reported at 10-30% in modern studies. Evisceration techniques include the Virchow method (organs removed individually), Rokitansky method (in situ dissection), and Letulle method (en bloc removal). Organ-specific dissection protocols, neuropathology examination after adequate fixation, ancillary testing (microbiology, toxicology, biochemistry, genetics), and structured autopsy reporting are integral components. Minimally invasive autopsy techniques, including post-mortem MRI and CT-guided tissue sampling, offer alternatives where consent for full autopsy is declined.
Standardised classification systems provide the shared language of pathology, ensuring consistent diagnosis, reproducible grading, and evidence-based staging that underpins clinical trial eligibility and treatment guidelines worldwide.
The WHO Classification of Tumours (WHO Blue Books) represents the international standard for tumour nomenclature and diagnostic criteria. The 5th edition series, published from 2019 onwards, integrates morphological classification with molecular biomarkers, recognising new entities defined by specific genetic alterations. Volumes cover CNS tumours, breast tumours, thoracic tumours, head and neck tumours, female genital tumours, urinary and male genital tumours, soft tissue and bone tumours, skin tumours, haematolymphoid tumours, digestive system tumours, and endocrine tumours. Each entity is defined by essential and desirable diagnostic criteria, ICD-O coding, and evidence-based grading when applicable. The classification is regularly updated to incorporate new molecular discoveries and revised diagnostic standards.
The TNM staging system, maintained jointly by the AJCC and UICC, provides the globally accepted framework for cancer staging. The T category describes primary tumour extent (size and local invasion), N describes regional lymph node involvement (number and location of positive nodes), and M describes distant metastasis. The 8th edition (2017) introduced significant changes including HPV-mediated oropharyngeal carcinoma staging, prognostic stage groups incorporating biomarkers in breast cancer, and Gleason score integration in prostate cancer staging. Pathological staging (pTNM) from surgical specimens provides the most accurate prognostic information and determines adjuvant therapy eligibility. Post-neoadjuvant staging (ypTNM) assesses treatment response and residual disease burden.
The Bethesda System provides standardised terminology and risk-stratified diagnostic categories for cytopathology reporting across multiple organ systems. The Bethesda System for Reporting Cervical Cytology (TBS) classifies results as negative for intraepithelial lesion or malignancy (NILM), ASC-US, ASC-H, LSIL, HSIL, and carcinoma, with each category linked to specific management algorithms. The Bethesda System for Reporting Thyroid Cytopathology categorises FNA results into six diagnostic categories (I-VI: non-diagnostic, benign, AUS/FLUS, follicular neoplasm, suspicious, malignant) with associated implied risks of malignancy guiding clinical management. Similar tiered reporting systems have been developed for pancreaticobiliary, salivary gland, and urinary cytology, each with defined risk of malignancy per category.
The Paris System for Reporting Urinary Cytopathology (TPS) provides a standardised, evidence-based framework for urine cytology reporting with defined diagnostic categories and associated risks of high-grade urothelial carcinoma (HGUC). Categories include non-diagnostic/unsatisfactory, negative for HGUC (NHGUC), atypical urothelial cells (AUC), suspicious for HGUC (SHGUC), HGUC, and low-grade urothelial neoplasm (LGUN). The system emphasises the primary role of urine cytology in detecting high-grade lesions, with morphological criteria centring on nuclear-to-cytoplasmic ratio, nuclear hyperchromasia, irregular nuclear membranes, and coarse chromatin. The second edition refined category definitions and addressed the incorporation of ancillary testing such as UroVysion FISH and immunocytochemistry.
The classification of haematolymphoid neoplasms has been updated in parallel systems: the WHO Classification of Haematolymphoid Tumours (WHO-HAEM5) and the International Consensus Classification (ICC), both published in 2022. These classifications define disease entities by integrating morphology, immunophenotype, cytogenetics, and molecular genetics. Key updates include refined subclassification of myelodysplastic syndromes by genetic driver (SF3B1, TP53 biallelic), recognition of new lymphoma entities, updated diagnostic criteria for myeloproliferative neoplasms based on WHO bone marrow histology criteria, and molecular refinement of acute myeloid leukaemia categories. The dual classification framework has generated debate, but both systems share the fundamental principle of genetically defined disease classification.
Numerous organ-specific and disease-specific classification and grading systems standardise pathological reporting. These include the Nottingham grading system for breast carcinoma (tubule formation, nuclear pleomorphism, mitotic count), ISUP Grade Groups for prostate adenocarcinoma (replacing Gleason score reporting), the IASLC grading system for lung adenocarcinoma, the Milan System for reporting salivary gland cytopathology, the Royal College of Pathologists datasets for synoptic cancer reporting, the CAP cancer protocols providing checklist-based structured reporting for each tumour type, the Banff classification for transplant pathology, the ISN/RPS classification for lupus nephritis, and the Marsh-Oberhuber classification for coeliac disease. Adoption of structured synoptic reporting enhances completeness and consistency compared with narrative reports.
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