Histopathology and Cytology
Histopathology and cytology tests specimen handling from theatre to slide: fixation, processing, staining, and the limits of frozen section and fine-needle aspiration.
Core rules
- Fix in 10% neutral buffered formalin at roughly ten times the specimen volume; fixation delay and under-fixation are the commonest causes of poor morphology.
- Processing: dehydration in graded alcohols, clearing in xylene, impregnation and embedding in paraffin; section at 3–5 micrometres.
- Haematoxylin stains nuclei blue; eosin stains cytoplasm and collagen pink. Special stains: PAS for glycogen and fungi, Ziehl–Neelsen for acid-fast bacilli, reticulin for architecture, Perls for iron.
- Immunohistochemistry uses antigen–antibody reactions with a chromogen; cytokeratin marks epithelial cells, vimentin mesenchyme, CD45 leucocytes.
- Cytology: Pap stain for cervical smears, Giemsa for aspirates; report cervical samples using Bethesda categories from NILM to HSIL and carcinoma.
Worked example
A sentinel node is sent for frozen section during breast surgery. The cryostat section shows metastatic epithelial cells, reported within 20 minutes, and the surgeon proceeds to axillary clearance. The reasoning: frozen section is fast but less sensitive than paraffin sections and immunohistochemistry, so a negative frozen result does not exclude micrometastasis and permanent sections remain the final report.
Traps
Placing a specimen in formalin when microbiology or flow cytometry was requested; allowing air-drying artefact before fixation; and reporting a cytology smear as negative when the sample was acellular or poorly preserved.