Pleural Fluid Cytology and Malignant Mesothelioma

Pleural fluid cytology is the examination of cells in fluid collected from the pleural cavity around the lungs. Its principal diagnostic challenges are to determine whether an effusion is malignant and, when malignant cells are mesothelial in origin, to distinguish reactive mesothelial proliferation from malignant mesothelioma and from metastatic adenocarcinoma — a separation that often requires ancillary immunocytochemistry and molecular testing.

Definition

Pleural fluid cytology is the microscopic and ancillary study of cells in pleural effusions to classify the fluid as benign or malignant and, in mesothelial malignancy, to confirm malignant mesothelioma and separate it from reactive mesothelium and metastatic carcinoma.

Scope

The entry covers the cytomorphology of pleural effusions, the biochemical separation of transudates from exudates, and the diagnostic approach to malignant mesothelioma in effusion samples, including the ancillary markers (such as BAP1 loss, p16/CDKN2A homozygous deletion, and claudin-4) used to support a diagnosis. It is a reference on diagnostic interpretation and does not provide treatment guidance.

Core questions

• Is a pleural effusion a transudate or an exudate, and is it malignant?
• How can malignant mesothelioma be distinguished from reactive mesothelial proliferation in effusion cytology?
• Which immunocytochemical and molecular markers reliably separate mesothelioma from metastatic adenocarcinoma?

Key concepts

• Transudate versus exudate (Light's criteria)
• Reactive mesothelial proliferation
• Malignant mesothelioma
• Metastatic adenocarcinoma in effusions
• BAP1 loss and p16/CDKN2A homozygous deletion
• Claudin-4 immunocytochemistry
• Cell-block preparation and ancillary testing

Mechanisms

Pleural effusions accumulate when serous fluid production exceeds absorption across the pleura, whether from systemic pressure imbalance (transudate) or local inflammation, infection, or tumor (exudate); Light's criteria operationalize this distinction biochemically. Malignant cells reach the pleura by direct extension, lymphatic or hematogenous spread, or, in mesothelioma, by primary transformation of the mesothelial lining. Because reactive mesothelial cells can appear atypical, ancillary tests are central: loss of BAP1 nuclear expression and homozygous deletion of p16 (CDKN2A) by FISH support a diagnosis of mesothelioma over reactive change, while claudin-4 positivity favors adenocarcinoma over mesothelial origin. These markers are interpreted together with morphology rather than in isolation.

Clinical relevance

A malignant pleural effusion is frequently the specimen that first establishes pleural involvement by cancer, and the distinction between mesothelioma, metastatic carcinoma, and a benign reactive process has substantial diagnostic and staging implications. This entry describes how such determinations are made cytologically and is not a basis for individual diagnostic or therapeutic decisions.

Epidemiology

Malignant mesothelioma is strongly associated with prior asbestos exposure and typically presents decades after exposure, often with a pleural effusion. Metastatic carcinoma, however, is a far more common cause of malignant pleural effusion overall, which is why ancillary marker panels that separate mesothelial from epithelial malignancy are routinely applied.

Evidence & guidelines

Ancillary panels combining BAP1 immunohistochemistry with p16 (CDKN2A) FISH have been shown to support the diagnosis of mesothelioma in effusion specimens, and claudin-4 has been reported as highly effective in distinguishing adenocarcinoma from mesothelioma. The International System for Reporting Serous Fluid Cytopathology provides standardized reporting categories and ancillary-testing recommendations applicable to pleural fluid.

History

The biochemical separation of pleural transudates from exudates was established by Light and colleagues in 1972 and remains a reference standard. The cytologic recognition of mesothelioma in effusions evolved as immunocytochemistry matured, and from the 2010s ancillary molecular markers — notably BAP1 loss and p16 deletion — were incorporated to improve the distinction from reactive mesothelial proliferation, a development reviewed as the field moved toward effusion-based diagnosis.

Debates

Can malignant mesothelioma be diagnosed on effusion cytology alone?

Effusion cytology supplemented by ancillary markers such as BAP1 loss and p16 deletion can support a mesothelioma diagnosis, but the reliability of a cytology-only diagnosis versus a requirement for tissue biopsy remains a point of practice variation.

Key figures

• Richard W. Light
• Andrew Churg
• Edmund S. Cibas
• Ajit Paintal

Seminal works

• light-1972
• hwang-2016
• jo-2014-claudin4

Frequently asked questions

How is malignant mesothelioma distinguished from reactive mesothelial cells in pleural fluid?

Morphologic features are combined with ancillary tests; loss of BAP1 nuclear staining and homozygous deletion of p16 (CDKN2A) by FISH support malignancy, whereas reactive mesothelial cells typically retain these markers.

What distinguishes mesothelioma from metastatic adenocarcinoma in an effusion?

Immunocytochemical markers are used; for example, claudin-4 is typically positive in adenocarcinoma and negative in mesothelioma, helping separate epithelial metastases from a primary mesothelial malignancy.