When fluid builds up in a normally sterile part of the body, like a joint (synovial fluid) or the abdomen (ascitic fluid), this test provides the definitive answer. Your doctor will collect a small sample of this fluid. Our lab then cultures it to see if any aerobic bacteria (germs that need oxygen) grow. Finding bacteria in these fluids is a critical, high-alert finding. This test is essential for diagnosing the exact cause of the infection (like septic arthritis), so your doctor can prescribe the most targeted, effective antibiotic treatment immediately.