What is the primary laboratory finding that distinguishes a cardiac source for ascites?

The serum-ascites albumin gradient (SAAG) is a valuable tool in determining the cause of ascites. A gradient of 1.1 g/dL or greater indicates that the ascitic fluid is transudative, which typically suggests that the ascites is due to conditions such as portal hypertension that may arise from cardiac issues, liver cirrhosis, or nephrotic syndrome. In cases of cardiac ascites, which is often due to heart failure, the fluid tends to have a low protein content and thus shows a SAAG of 1.1 g/dL or more.

In contrast, a lower SAAG, particularly less than 1.1 g/dL, would point towards conditions that are typically associated with an exudative process, such as infections or malignancies. This makes the SAAG an important distinguishing factor in identifying cardiac disease as a potential contributor to ascitic fluid accumulation.

The other findings listed, such as total protein levels, white blood cell counts, and pH levels can provide additional information about the nature of the ascitic fluid but do not specifically indicate a cardiac source for ascites in the manner that a high SAAG does. Total protein levels may suggest different underlying conditions but are