When fluid accumulates in the body (e.g., in the abdomen or around the lungs), this test is essential to find the cause. It measures the amount of albumin (a key protein) in that specific fluid sample. Your doctor compares this to your blood albumin level to determine why the fluid is there. This is the gold-standard method for distinguishing between causes like liver cirrhosis (which leads to low-protein fluid) and inflammation or infection. This test provides a clear diagnostic pathway, answering the critical question of "why" and guiding your treatment.