Widal test and its fallacy- an editorial

Abstract

Enteric Fever is a fatal multisystem illness caused by genus Salmonella. Typhoid fever is caused by Salmonella typhi and paratyphoid fever by Salmonella paratyphi A and B. Blood culture is the gold standard for diagnosis of Enteric fever, and also bone marrow or stool culture¹. When facilities for culturing are not available, the Widal test is the reliable and can be of value in the diagnosis of Enteric fevers in endemic areas.

The Widal test is a serological test for diagnosis of Enteric fever. It was developed in 1896 and named after its inventor, Georges-Fernand Widal.  

Widal test is an indirect agglutination test which detects the presence of serum agglutinins (O and H) in patients’ serum with typhoid and paratyphoid fever. The organisms causing enteric fever possesses two major antigens namely somatic antigen (O) and a flagellar antigen (H) along with another surface antigen, Vi.

During infection with typhoid or paratyphoid bacilli, antibodies against somatic antigen of S.typhi (O), flagellar antigen of S. typhi (H), S. paratyphi A (AH), and S. paratyphi B (BH) usually become detectable in blood 6 days after the onset of infection. O antibody appears first, decline after recovery & disappears in 3-6 months. H antibody appears slightly later but persists longer and can be used to distinguish between various types of enteric fever². 

Widal test measures the antibody titre (O and H) using various dilution of Salmonella antigen suspensions by either slide method or tube method.

Baseline titre is 1:40. A single Widal test is of little clinical relevance especially in endemic areas such as Indian subcontinent, Africa and South-east Asia. This is due to recurrent exposure to the Salmonella bacteria, immunization and high chances of cross-reaction from infections, such as malaria and non typhoidal salmonella^3,4.  

A 4 fold rise or a rising titer in paired samples at an interval of 7-10 days suggests the serological diagnosis of enteric fever.  

High titer > 1:160 against O antigen and high titer > 1:160 against H antigen suggest typhoid fever.