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What is a widal test done for?

Q: I am a doctor. I could find in a widal test result TO 1:80, TH 1:40, AH and BH are negative. What does this indicate? What is TO, TH, AH and BH?

A:The Widal test is a blood test, widely used for serological diagnosis of enteric fever (typhoid / paratyphoid fever). It measures the titre (level) of antibodies (in blood of patients) against salmonella antigens:

  • TO – Somatic (O) antigen of Salmonella typhi
  • TH – Flagellar (H) antigen of Salmonella typhi
  • AH – Flagellar (H) antigen of Salmonella Paratyphi A
  • BH - Flagellar (H) antigen of Salmonella Paratyphi B The antibodies against O antigen of only Salmonella typhi are measured, since all Salmonella have similar O antigens. An increasing O antibody level signifies acute infection, whilst a raised H antibody level may indicate the serotype of the infecting organism, or an earlier infection. However, as for any other serological test, it has many limitations as a diagnostic tool, since it provides indirect evidence (as a surrogate marker) for infection. Also, the rise in antibody levels, needed to make the diagnosis, takes 7-14 days, which limits its use. The interpretation of the serology thus requires consideration of factors such as previous immunisation, the stage of the illness and the effect of any antibiotic treatment, infection with any other gram-negative bacteria, etc. As a diagnostic marker, Widal test must be done on paired serum samples. The first sample is taken early in the disease and the second sample at least 2 weeks later. A four-fold rise in antibodies in the paired samples is significant and diagnoses acute recent infections. Widal test can also be used in carrying out serological surveys in a community to know the prevalence (and endemicity in an area) of enteric fever and determine the cut off titre for interpretation in that population. The traditional Widal test is done in tubes (tube agglutination test) and needs 15-18 hours for the test to be done. A modified Widal test is commercially available as a slide agglutination test. However, the isolation of Salmonella bacteria in the blood (by blood culture) is the gold standard for diagnosing enteric fever.

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