The Widal Serum test is the classic, foundational blood test for diagnosing Typhoid (enteric) fever. It detects 'agglutinating' (clumping) antibodies (O and H) that your immune system produces against the Salmonella bacteria. This test is a critical diagnostic tool if you are experiencing a persistent, high fever. For a definitive diagnosis, this test often requires two samples taken 7-10 days apart to show a "rising titre," or antibody level. A simple blood draw provides this essential, time-tested diagnostic information, helping your doctor confirm the cause of your fever.