Transudat

Transudat, or transudate, is a non-inflammatory fluid that accumulates in body cavities as a result of systemic imbalances in Starling forces across capillary walls. It is an ultrafiltrate of plasma with relatively low protein content and few cells, in contrast to exudates that arise from inflammation, infection, or malignancy and contain higher protein and LDH as well as more cells.

Pathophysiology and causes

Transudates form when there is increased hydrostatic pressure in the capillaries or decreased plasma oncotic pressure. Common mechanisms include congestive heart failure and portal hypertension (increasing hydrostatic pressure) or hypoalbuminemia from nephrotic syndrome, cirrhosis, or severe malnutrition (lowering oncotic pressure). These processes can lead to fluid accumulation in serous cavities such as the pleural space, peritoneal cavity, or pericardial space.

Sites and clinical relevance

Pleural effusions, ascites, and pericardial effusions are the most typical manifestations. Transudates are usually clear and low in protein and cells, and they often reflect systemic rather than local inflammatory disease.

Diagnosis and laboratory features

For pleural effusions, Light’s criteria help distinguish transudates from exudates: pleural fluid to serum protein ratio <0.5, pleural fluid to serum LDH ratio <0.6, and pleural LDH <2/3 of the upper limit of normal. For ascites, the serum-ascites albumin gradient (SAAG) ≥1.1 g/dL suggests ascites due to portal hypertension (a transudative process), whereas SAAG <1.1 g/dL points toward non-portal hypertensive causes.

Management

Treatment focuses on the underlying condition (e.g., optimized heart failure therapy, diuretics and sodium restriction for cirrhosis-related ascites). Symptomatic drainage may be considered for large or burdensome effusions, with careful monitoring for complications.