Benign, Reactive, and Inflammatory Serous Effusions

Benign, reactive, and inflammatory serous effusions are non-malignant fluids that collect in the pleural, peritoneal, and pericardial cavities in response to systemic pressure imbalance, infection, inflammation, or serosal injury. Their cytologic importance lies less in any single diagnosis than in the challenge of distinguishing the often-striking atypia of reactive mesothelial cells from genuine malignancy — a central source of diagnostic difficulty and false-positive risk in effusion cytology.

Definition

Benign, reactive, and inflammatory serous effusions are non-neoplastic cavity fluids whose cytology shows mesothelial cells, macrophages, and inflammatory cells in patterns that must be distinguished from malignancy, often through morphology supported by ancillary immunocytochemistry.

Scope

The entry covers the cytomorphology of non-malignant effusions, the spectrum of reactive mesothelial change, the cellular patterns of acute and chronic inflammatory and infectious effusions, and the ancillary tests used to confirm a benign interpretation or to resolve atypical findings. It is a reference on diagnostic interpretation and does not provide treatment guidance.

Core questions

• How can reactive mesothelial proliferation be reliably distinguished from malignancy in an effusion?
• What cellular patterns characterize transudative, inflammatory, and infectious benign effusions?
• When findings are atypical but not clearly malignant, how should the specimen be categorized and worked up?

Key concepts

• Reactive mesothelial proliferation
• Transudate versus exudate
• Macrophages and histiocytes in effusions
• Lymphocyte-rich and neutrophil-rich effusions
• Atypia of undetermined significance
• Immunocytochemistry confirming benign mesothelium
• False-positive risk in effusion cytology

Mechanisms

Benign effusions arise when the balance of fluid production and absorption across a serous membrane is disturbed. Transudates result from systemic hydrostatic or oncotic pressure changes, as in heart failure or cirrhosis, and are typically paucicellular; exudates result from local inflammation, infection, or serosal injury that increases capillary permeability, yielding richer and more varied cellularity. The mesothelial lining responds to almost any irritation by proliferating, enlarging, and acquiring nuclear features — prominent nucleoli, multinucleation, and cell clustering — that can closely mimic adenocarcinoma. Inflammatory effusions show characteristic populations: neutrophil-predominant fluid in acute or septic processes, lymphocyte-rich fluid in tuberculous and some chronic effusions, and abundant macrophages in long-standing or hemorrhagic effusions. Distinguishing reactive mesothelium from malignancy relies on the principle that benign mesothelial cells form a morphologic continuum and retain mesothelial immunophenotype, whereas a second, foreign cell population or epithelial markers such as claudin-4 favor metastatic carcinoma.

Clinical relevance

Most effusions submitted for cytology are benign, and correctly recognizing reactive and inflammatory patterns avoids false-positive diagnoses that could misclassify a patient as having cancer. When findings are equivocal, standardized categories such as atypia of undetermined significance communicate uncertainty and prompt ancillary testing. This entry describes diagnostic reasoning and is not guidance for managing an individual patient.

Epidemiology

Benign causes — heart failure, cirrhosis, infection, and inflammation — account for a large share of effusions encountered in cytology practice. The proportion that are benign versus malignant varies by clinical setting, and the frequency of indeterminate or atypical interpretations is one reason standardized reporting categories with risk-of-malignancy estimates were developed.

Evidence & guidelines

The International System for Reporting Serous Fluid Cytopathology defines a 'negative for malignancy' category and an 'atypia of undetermined significance' category, with guidance on when ancillary testing is warranted to resolve reactive versus malignant change. Light's criteria provide the biochemical separation of transudates from exudates, and immunocytochemical markers such as claudin-4 help confirm that atypical cells are reactive mesothelium rather than adenocarcinoma.

History

The tendency of reactive mesothelial cells to mimic malignancy was recognized early in the development of effusion cytology and has long been described as one of the discipline's principal diagnostic pitfalls. The benign and atypical ends of the diagnostic spectrum were formalized in 2020 within The International System for Reporting Serous Fluid Cytopathology, which introduced explicit categories and risk estimates for non-malignant and indeterminate effusions.

Debates

How should atypical-but-not-malignant effusions be categorized?

The boundary between a benign reactive interpretation and 'atypia of undetermined significance' is judgment-dependent, and the criteria for assigning the atypical category and triggering ancillary testing continue to be refined.

Key figures

• Richard W. Light
• Ashish Chandra
• Barbara Crothers
• Edmund S. Cibas

Seminal works

• chandra-2020-brescia
• light-1972

Frequently asked questions

Why are reactive mesothelial cells a problem in effusion cytology?

Reactive mesothelial cells can become enlarged, multinucleated, and clustered with prominent nucleoli, closely resembling adenocarcinoma; recognizing this benign continuum and using ancillary markers helps avoid a false-positive diagnosis of malignancy.

What does an 'atypia of undetermined significance' result mean for an effusion?

It is a standardized reporting category used when cells show some atypical features but are not clearly malignant; it signals diagnostic uncertainty and often prompts ancillary testing or repeat sampling rather than a definitive call.