This test measures the glucose level in a sample of body fluid, such as fluid from around the lungs (pleural) or in the abdomen (ascitic). This is a specialized diagnostic test to help your doctor find the cause of an abnormal fluid buildup. Low glucose levels in the fluid often suggest an infection (as bacteria are consuming the glucose) or an inflammatory condition. A sample of the specific fluid is required, providing critical clues to help your doctor pinpoint the cause of your condition.