At what titre is a Widal test considered positive for Salmonella Typhi?

Widal Test Interpretation for Salmonella Typhi Diagnosis

A Widal test for Salmonella Typhi is generally considered positive at a titer of ≥1:160 for both O and H antigens, though interpretation must consider local endemicity patterns.

Diagnostic Value of Widal Test

The Widal test measures agglutinating antibodies against O (somatic) and H (flagellar) antigens of Salmonella Typhi. Proper interpretation requires understanding several key factors:

Standard Diagnostic Titers

• O antigen: A titer of ≥1:160 is generally considered positive 1, 2
• H antigen: A titer of ≥1:160 is generally considered positive 2, 3

Sensitivity and Specificity Considerations

• At a cutoff titer of 1:160 for O antigen:

  • Sensitivity: 58-61.2% in bacteriologically confirmed typhoid fever 1
  • Specificity: 87.3-89.8% when compared to non-typhoidal febrile illnesses 1, 2

• At a cutoff titer of 1:80 for both O and H antigens:

  • Sensitivity: 90%
  • Specificity: 87.3% for O antigen and 88.5% for H antigen 2

Regional Variations in Interpretation

Diagnostic titers may need adjustment based on local endemicity:

• In endemic areas like Sudan, higher background titers exist in healthy individuals:

  • Up to 10.5% of healthy individuals may have O antigen titers of 1:320 4
  • A higher cutoff of >1:320 has been suggested in such regions 4

• In Ethiopia, titers ≥1:160 for either H or O antigens in unvaccinated patients with typhoid-like symptoms are strongly suggestive of typhoid fever 3

Timing Considerations

• H antigen titers typically rise earlier and more frequently than O antigen titers 3
• Antibody rise is maximal during the second week of illness 3
• A single Widal test has limited value; paired sera showing a four-fold rise in titer is more reliable 5

Limitations and Pitfalls

1. False positives: May occur in:

   • Other febrile illnesses (6.4-11% positive at 1:160 titer) 6
   • Previous typhoid vaccination
   • Cross-reactions with non-typhoidal Salmonella

2. False negatives: May occur in:

   • Early disease (first week)
   • Prior antibiotic treatment
   • Immunocompromised patients

3. Regional variations: Background titers in healthy populations vary geographically, requiring locally adjusted cutoffs

Clinical Application

When interpreting Widal test results:

1. Consider the clinical presentation alongside laboratory results
2. In endemic areas, use higher cutoff titers (>1:320)
3. In non-endemic areas, titers of ≥1:160 are more significant
4. When possible, obtain paired sera 10-14 days apart to demonstrate rising titers
5. Blood cultures remain the gold standard for diagnosis of typhoid fever

Alternative Diagnostic Approaches

When available, consider more specific diagnostic methods:

• Blood cultures (highest yield within first week of symptoms) 5
• Bone marrow cultures (higher sensitivity than blood cultures) 5
• Newer rapid serological tests (Typhidot, Tubex) may offer improved specificity 5

Remember that the Widal test should be interpreted in the context of the clinical picture, as the clinical presentation remains the decisive factor in diagnosing typhoid fever when laboratory results are equivocal 3.