13/01/2026
Widal Test
(Educational Purpose Only)
Objective
To detect agglutinating antibodies (O and H) against Salmonella typhi and Salmonella paratyphi in the patient’s serum for the diagnosis of enteric (typhoid) fever.
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Principle
The Widal test is based on the agglutination reaction between Salmonella antigens (O and H) and their corresponding antibodies present in the patient’s serum.
When antigen and antibody react, visible clumping (agglutination) occurs, indicating a positive reaction.
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Materials
• Patient serum sample
• Widal antigen suspensions (O, H, AH, BH)
• Glass slide or test tubes
• Normal saline
• Mixing sticks / droppers
• Water bath (for tube test)
• Incubator
• Personal protective equipment
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Procedure (Point Wise)
A. Slide Widal Test
1. Place a drop of patient serum on a clean glass slide.
2. Add one drop of specific Widal antigen.
3. Mix gently using a stick.
4. Rock the slide for 1 minute.
5. Observe for agglutination.
B. Tube Widal Test
1. Prepare serial dilutions of patient serum in test tubes.
2. Add equal volume of specific antigens to each tube.
3. Incubate at 37°C for 16–18 hours.
4. Observe for agglutination.
5. Highest dilution showing agglutination is taken as the titer.
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Result
• Positive: Visible agglutination present
• Negative: No agglutination
Significant Titer (may vary by region):
• O antigen ≥ 1:80
• H antigen ≥ 1:160
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Uses
• Diagnosis of typhoid fever
• Screening in endemic areas
• Epidemiological studies
• Supporting clinical diagnosis
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Conclusion
The Widal test is a simple and economical serological test for detecting antibodies against Salmonella. However, results must be interpreted carefully along with clinical findings and other laboratory tests due to possible false positives and negatives.
