Q: What is the Widal test and how accurate it is for diagnosing enteric fever?

A:Widal Test is a serological test widely used for diagnosis of enteric fever. However, as for any other serological test, it has many limitations as a diagnostic tool. Its main use is in carrying out sero-surveys in a community to know the endemicity of any infection. While using widal test for the diagnosis of enteric fever, several factors need to be considered for interpretation. 1. Endemicity of enteric fever in an area of investigation. This would determine a cut off titre in that population. 2. Administration of antibodies to the patient will effect te result. 3. Immunization with any typhoid vaccine or a previous infection/exposure will affect the test. 4. The stage of the disease at the time of collection of sample. Early in the disease low antibody titres are found. The antibodies start rising after first week of illness and do so until third-fourth week. Thereafter the titers start falling. 5. Infection with any other gram-negative bacteria may give a false positive reaction. To make any logical conclusion in the diagnosis of enteric fever on the basis widal test, one must submit a paired serum sample. The first sample taken early in the disease and the second sample atleast 2 weeks later. Demonstration of a four-fold rise in antibodies in the paired samples may diagnose acute recent infections. Modified widal test is a commercially available slide agglutination test in which IgM antibodies can be detected. The presence of IgM is indicative of acute recent infections. This test needs to be evaluated using bacterial isolation as gold standard.