This is a highly specialized test to determine the cause of fluid buildup in the body (e.g., around the lungs or heart). It measures the level of LDH, a general enzyme, in that specific fluid sample. By comparing the fluid's LDH level to the blood's LDH level, your doctor can determine if the fluid is caused by inflammation and tissue damage (high LDH) or another issue like pressure changes (low LDH). A sample of the fluid is required, providing the definitive clue your doctor needs to diagnose the underlying condition.