Widal test is a type of

Tube dilution agglutination test (Widal test): Serum agglutinins rise sharply during the second and third weeks of S serotype Typhi infection. The Widal test to detect these antibodies against the O and H antigens has been in use for decades. At least two serum specimens, obtained at intervals of 7-10 days, are needed to prove a rise in antibody titer. Serial dilutions of unknown sera are tested against antigens from representative salmonellae. The interpretive criteria when single serum specimens are tested vary, but a titer against the O antigen of greater than 1:320 and against the H antigen of greater than 1:640 is considered positive.Ref: Jawetz Medical Microbiology; Chapter15 Enteric Gram-Negative Rods (Enterobacteriaceae); Page no: 241