transudate

Transudates form when there is disruption of Starling forces governing fluid movement across capillary walls—either increased hydrostatic pressure (e.g., congestive heart failure, venous obstruction) or decreased oncotic pressure (e.g., hypoalbuminemia from nephrotic syndrome, liver cirrhosis, protein malnutrition). Because the capillary endothelium remains intact and there is no significant inflammatory process, transudates have low protein content (<3 g/dL), low LDH, and few cells. In contrast, exudates result from increased vascular permeability due to inflammation (e.g., infection, malignancy, autoimmune disease) and are protein-rich, cellular, and often cloudy.

Clinically, Light's criteria are used to differentiate pleural transudates from exudates: a transudate has pleural fluid/serum protein ratio <0.5, pleural fluid/serum LDH ratio <0.6, and pleural LDH less than 2/3 the upper limit of normal serum LDH. Common clinical scenarios producing transudative effusions include CHF (most common cause of transudative pleural effusion), cirrhosis with ascites (transudative due to portal hypertension and hypoalbuminemia), and nephrotic syndrome. Peripheral edema from transudates is typically seen in heart failure (pitting edema), liver disease, and renal disease.

Understanding the transudate-exudate distinction is essential for diagnostic workup of effusions (pleural, peritoneal, pericardial) and helps narrow differential diagnoses—guiding whether to pursue infectious/inflammatory/malignant workup (exudate) versus systemic causes affecting fluid balance (transudate).