Peritoneal and Ascitic Fluid Cytology

Peritoneal and ascitic fluid cytology is the examination of cells in fluid that accumulates in the peritoneal cavity (ascites) and in saline washings of the peritoneal surface. It is used to detect malignant involvement of the abdominal cavity — most often by metastatic adenocarcinoma — and, in gynecologic and gastrointestinal cancers, peritoneal washing cytology contributes to surgical staging.

Definition

Peritoneal and ascitic fluid cytology is the microscopic and ancillary examination of cells from peritoneal effusions and washings to classify the specimen as benign or malignant and, where malignant, to help identify the primary tumor.

Scope

The entry covers the cytomorphology of peritoneal fluids and washings, the biochemical and clinical triage of ascites (including the serum-ascites albumin gradient that separates portal-hypertensive from non-portal causes), and the ancillary techniques used to identify and characterize malignant cells. It is a reference on diagnostic interpretation rather than clinical management guidance.

Core questions

• Does ascitic fluid or a peritoneal washing contain malignant cells?
• How does the serum-ascites albumin gradient help separate portal-hypertensive ascites from malignant or other causes?
• How are reactive mesothelial cells distinguished from metastatic adenocarcinoma in peritoneal specimens?

Key concepts

• Ascites and its causes
• Serum-ascites albumin gradient (SAAG)
• Peritoneal washing cytology and staging
• Reactive mesothelial cells versus adenocarcinoma
• Psammoma bodies and serous carcinoma
• Cell-block and immunocytochemistry
• The International System for Reporting Serous Fluid Cytopathology

Mechanisms

Ascites develops when peritoneal fluid production exceeds reabsorption. In cirrhosis the dominant mechanism is portal hypertension with sodium and water retention, producing a high serum-ascites albumin gradient; malignant, infectious, and inflammatory causes more often produce a low gradient. The serum-ascites albumin gradient, introduced by Runyon and colleagues, reflects the oncotic pressure difference across the peritoneal capillaries and has largely supplanted the older exudate-transudate dichotomy for ascites. When tumor seeds the peritoneum, malignant cells exfoliate into the fluid or are captured in washings; distinguishing them from reactive mesothelial cells — which proliferate readily in response to any peritoneal irritation — relies on morphology supported by immunocytochemistry, such as epithelial markers and claudin-4 for adenocarcinoma versus mesothelial markers for reactive cells.

Clinical relevance

Peritoneal fluid and washing cytology can establish peritoneal spread of malignancy and, in some gynecologic and gastrointestinal cancers, positive washing cytology has staging significance. The serum-ascites albumin gradient is a standard part of the laboratory work-up of new ascites. This entry explains how such specimens and tests are interpreted and is not a guide to managing an individual patient.

Epidemiology

Most ascites in adults is due to cirrhosis and portal hypertension, with malignancy, heart failure, and infection accounting for much of the remainder. Among malignant causes, metastatic adenocarcinoma — including ovarian, gastrointestinal, and breast primaries — predominates, which shapes the ancillary marker panels applied to peritoneal specimens.

Evidence & guidelines

The serum-ascites albumin gradient was validated by Runyon and colleagues as superior to the exudate-transudate concept for classifying ascites, and gradient-based assessment is incorporated into the American Association for the Study of Liver Diseases practice guideline on ascites due to cirrhosis. For cytologic reporting, The International System for Reporting Serous Fluid Cytopathology provides standardized categories applicable to peritoneal fluid.

History

Peritoneal fluid examination developed as part of exfoliative cytology, and peritoneal washing cytology became established in the surgical staging of gynecologic and gastrointestinal cancers. The clinical classification of ascites was reframed in 1992 when Runyon and colleagues showed the serum-ascites albumin gradient to be a more accurate discriminator than the transudate-exudate distinction, an approach later embedded in hepatology practice guidelines.

Key figures

• Bruce A. Runyon
• Edmund S. Cibas
• Ashish Chandra

Seminal works

• runyon-1992-saag
• chandra-2020-brescia

Frequently asked questions

What is the serum-ascites albumin gradient used for?

It is calculated by subtracting the ascitic-fluid albumin from the serum albumin and is used to classify ascites; a high gradient indicates portal hypertension, while a low gradient points toward malignant, infectious, or other non-portal causes.

Why are peritoneal washings examined in cancer surgery?

Saline washings of the peritoneal cavity are examined for malignant cells because their presence can indicate peritoneal spread and, in certain cancers, contributes to surgical staging.