This is a highly specialized, diagnostic test performed on a sample of body fluid (such as abdominal 'ascitic' fluid) rather than blood. It measures the level of AFP, a protein that is normally high in a developing fetus but low in adults. An elevated level of AFP within a specific body fluid is a strong indicator that a tumor (like a liver cancer) may be present in that exact area. This direct test provides critical, localized information, giving your doctor a clear view of what is happening at the site of fluid buildup.