OncologyTier 2Topic

Body fluid analysis, including cytology

Role

Cytological examination of body fluids (pleural, peritoneal/ascitic, pericardial, CSF, urine) to detect malignant cells - a key, relatively non-invasive step in diagnosing and staging malignancy presenting with an effusion or fluid collection
A malignant effusion (pleural or peritoneal) often signals advanced/incurable disease and carries significant prognostic weight when found

Malignant cells shed into a body cavity either from direct tumour seeding of the serosal surface (e.g. peritoneal carcinomatosis from ovarian/gastric cancer, pleural involvement from lung/breast cancer) or occasionally from a primary malignancy of the cavity lining itself (mesothelioma)

Sensitivity of a single cytology sample is imperfect (~60-70% for a clearly malignant effusion) - repeat sampling improves yield; a negative cytology does not exclude malignancy if clinical suspicion remains high
Cell block preparation + immunohistochemistry on the cytology specimen can often identify the likely tissue of origin, similar to a tissue biopsy, without needing a separate invasive procedure
Mesothelioma cytology can be difficult to distinguish from reactive mesothelial cells or metastatic adenocarcinoma - often needs additional immunohistochemical markers or tissue biopsy for confirmation

A confirmed malignant effusion generally changes staging (upstages many solid tumours to advanced/metastatic disease) and management focus toward symptom control (drainage, pleurodesis) alongside any systemic therapy
Repeat sampling or escalation to image-guided/thoracoscopic biopsy is warranted when cytology is negative but clinical suspicion of malignancy remains high

Malignant pleural effusion, peritoneal carcinomatosis, mesothelioma, CSF cytology in suspected leptomeningeal disease

Confirmed malignant cytology in an effusion is generally associated with a poor prognosis and often reframes management toward palliative-focused, symptom-directed care

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