Cytopathology is the study of individual cells retrieved from the body, either by spontaneous exfoliation (shedding) or by active sampling (aspiration). It provides rapid, minimally invasive diagnosis for a wide range of conditions, particularly cancer screening and confirmation.
Exfoliative vs. Aspiration Cytology
Exfoliative cytology examines cells that are shed naturally from epithelial surfaces or collected by lavage, brushing, or scraping. Common specimens include cervical Pap smears (exfoliated cervical cells), urine (urothelial cells shed into urine), sputum and bronchoalveolar lavage (respiratory epithelial cells), and effusion fluids (pleural, peritoneal, pericardial — mesothelial cells and exfoliated tumor cells).
Fine needle aspiration (FNA) cytology uses a thin needle (22-27 gauge) to aspirate cells from a solid or cystic lesion. FNA can target palpable lesions (thyroid, breast, lymph node, salivary gland) or deep-seated lesions under ultrasound or CT guidance (lung, liver, pancreas, kidney, mediastinum). FNA is less invasive than core needle biopsy, produces immediate results via rapid on-site evaluation, and has lower complication rates.
Specimen Collection and Preparation
Conventional smears — the aspirated material is expelled onto glass slides and smeared using a second slide (pull or push smear), or spread with a needle (blood film technique). Smears are either air-dried (for Romanowsky-type stains: Diff-Quik, Giemsa) or fixed immediately in 95% ethanol (for Papanicolaou stain). Immediate fixation prevents air-drying artifact in Papanicolaou-stained smears.
Liquid-based cytology (LBC) — the sample is rinsed into a preservative solution, processed in an automated instrument that disperses cells and deposits a thin, uniform layer on a slide. LBC reduces obscuring blood and inflammation, produces cleaner backgrounds, and allows residual sample to be used for molecular testing. LBC is the standard for cervical cytology (ThinPrep, SurePath) and is increasingly used for non-gynecologic specimens.
Cell blocks — the residual tissue fragments and cells in the FNA needle rinse or LBC vial are concentrated by centrifugation, embedded in agar or plasma-thrombin clot, and processed as a paraffin block. Cell blocks provide sections for H&E, IHC, and molecular studies, maximizing diagnostic yield from small samples.
Staining in Cytopathology
Papanicolaou (Pap) stain is the standard stain for gynecologic and non-gynecologic cytology. It differentially stains cytoplasm (orange, green, blue depending on keratinization and cell type) and nuclei (blue-black with crisp chromatin detail). The Pap stain is excellent for evaluating nuclear features — the most important criteria for malignancy in cytology.
Romanowsky stains (Diff-Quik, Giemsa, Wright) are used for air-dried smears. They stain cytoplasm blue-purple, nuclei purple, and highlight cytoplasmic granules, vacuoles, and extracellular material (colloid, mucin, matrix). Romanowsky stains are superior for evaluating hematolymphoid cells, sarcoma, and melanoma.
Reporting and Classification
Cytology reports use standardized terminology systems that communicate the risk of malignancy. The Bethesda System for Cervical Cytology classifies squamous lesions as ASC-US, ASC-H, LSIL, HSIL, and squamous cell carcinoma; glandular lesions as AGC and adenocarcinoma. The Bethesda System for Thyroid Cytology uses six categories (I-VI): nondiagnostic, benign, atypia of undetermined significance (AUS), follicular neoplasm, suspicious for malignancy, and malignant. Each category carries an implied risk of malignancy and a recommended management action.
Advantages and Limitations
Cytology is rapid (results in minutes for on-site evaluation, 1-2 days for routine cases), minimally invasive (needle smaller than biopsy needle), cost-effective, and highly specific for malignancy (specificity >95% for most sites). However, cytology has limited sensitivity for some lesions (false-negative rates of 5-20% depending on site), cannot always distinguish in situ from invasive carcinoma, and provides limited material for ancillary studies if the sample is scant. Integration with histology, IHC, and molecular testing maximizes diagnostic accuracy.
