What tests can confirm if the belly swelling is fluid from heart failure?

The tests that can confirm belly swelling is fluid from heart failure are straightforward, but the evidence here is thin on the specifics.

The standard approach starts with a physical exam and an ultrasound of the abdomen. A paracentesis, where a needle draws fluid from the belly, is the definitive test. The fluid is analyzed for protein, glucose, and pH. One study from 2013 found that a simple dipstick test on that fluid, using a formula based on protein, glucose, and pH, could tell transudate (fluid from heart failure or liver disease) from exudate (fluid from infection or cancer) with about 94% accuracy [5]. The reference standard they compared it to was the serum-ascites albumin gradient, or SAAG, which is the standard lab calculation. A SAAG above 1.1 g/dL points to portal hypertension, which heart failure causes by backing up pressure into the liver.

Blood tests are also key. BNP and NT-proBNP are peptides released by the heart when it is stretched. One study from 2010 showed that pleural fluid NT-proBNP levels were significantly higher in patients with congestive heart failure (median 4189 pg/mL) compared to other causes like malignancy (median 1232 pg/mL) [3]. Another study from 2005 found BNP was the only independent predictor of cardiac events in heart failure patients [2]. But BNP can also be elevated in cirrhosis, which can cause ascites on its own, so it is not a perfect test [4]. An echocardiogram, which measures ejection fraction, is the direct way to see if the heart is pumping poorly. One case report from 2005 described a patient with chylous ascites and an ejection fraction of 35% whose fluid resolved with diuretics [1].

The evidence here does not include a large trial comparing these tests head-to-head in patients with prior open-heart surgery and multiple stents. The studies are small, old, and from different populations. The 2013 dipstick study [5] is the most directly useful, but it was done in a general ascites population, not specifically in post-CABG patients. The BNP studies [2, 3] are from 2005 and 2010, and the case report [1] is from 2005. None of these studies were funded by manufacturers, but the evidence base is narrow.

My call: the combination of an abdominal ultrasound, a paracentesis with SAAG calculation, and a blood BNP or NT-proBNP level, plus an echocardiogram, is the standard workup to confirm if belly swelling is from heart failure. The evidence supports this approach but is not strong for this exact patient. Confidence: moderate.