Widal’s test /vēdäls″/ [Georges F. I. Widal, French physician, 1862–1929] , an agglutination test used to aid in the diagnosis of Salmonella infections such as typhoid fever. This test measures the level of cold or febrile agglutinins in the blood that causes red blood cells to stick together at low or high temperatures. A fourfold increase in titer of agglutinins to O or H antigens is highly suggestive of active infection. A high titer may persist for years after the disease or after immunization against typhoid fever.
