Infection Markers Widal Test

The Widal Test is a serological test that detects the presence of specific antibodies (agglutinins) in a patient’s blood against the O (somatic) and H (flagellar) antigens of Salmonella typhi and Salmonella paratyphi – the bacteria responsible for typhoid and paratyphoid fevers. These infections are transmitted through contaminated food or water and are common in areas with poor sanitation.

The test is especially useful in suspected cases of typhoid fever, particularly when blood culture facilities are not available or in later stages of the disease when cultures may turn negative.

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What the Widal Test Measures:

• Anti-O antibodies: Indicate an acute infection with Salmonella typhi.

• Anti-H antibodies: Indicate past infection or immunization.

• Anti-AH & Anti-BH: Related to Salmonella paratyphi A and B respectively.

The test is usually performed using a tube agglutination method or a slide agglutination test, where the patient’s serum is mixed with antigen suspensions to check for clumping (agglutination), indicating the presence of antibodies.

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Why It’s Done:

• Diagnose typhoid or paratyphoid fever

• Differentiate between current and past infections

• Monitor progression or response to treatment

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Test Procedure:

• Requires a blood sample

• Typically no fasting is needed

• Results available within a few hours

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Limitations:

• May give false positives in previously infected or vaccinated individuals

• Cross-reactivity with other infections is possible

• Best used in conjunction with clinical evaluation and other diagnostic tests

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Interpretation (Example):

• A titer of ≥1:160 for O or H antigen may suggest infection, but rising titers in repeated tests are more conclusive.

The Widal Test remains a valuable diagnostic aid for enteric fever, especially in low-resource settings. However, it should ideally be interpreted alongside clinical symptoms, travel history, and possibly blood cultures for accurate diagnosis.