When fluid accumulates in the body (e.g., in the lungs or abdomen), this test is critical to finding the why. It measures the total amount of protein (both albumin and globulins) in that specific fluid sample. This result is essential for your doctor to determine if the fluid is caused by pressure issues (like heart failure, which produces low-protein fluid) or by inflammation/infection (which produces high-protein fluid). A sample of the fluid is required, providing the definitive clue your doctor needs to make the right diagnosis.