Serology – interpreting the Widal test for typhoid In routine Widal testing, which single-serum titre for H agglutinins is generally considered significant in endemic settings (assuming appropriate baseline titres and clinical context)?

Introduction / Context:
The Widal test measures agglutinating antibodies to Salmonella Typhi (O and H antigens) and to Salmonella Paratyphi antigens. Interpretation depends on baseline titres in the population, timing of sampling, and clinical correlation, and is often misunderstood.

Given Data / Assumptions:

• Single-sample interpretation varies by region and laboratory cutoffs.
• In many Indian laboratories, a titre of 1:200 or higher for H is often taken as significant, while O cutoffs may be lower or similar depending on baseline.
• Paired sera demonstrating a fourfold rise remain the gold standard when feasible.

Concept / Approach:
H agglutinins (flagellar antibodies) generally rise slightly later than O antibodies and can persist. In endemic areas, low-level background titres are common; therefore, higher cutoffs are used to improve specificity. A single titre of 1:200 or more for H, in the right clinical window and with supportive O titres, increases the likelihood of active infection.

Step-by-Step Solution:
Recognize endemic context with background seropositivity.Apply conservative single-sample threshold: H ≥ 1:200.Note that paired sera with a fourfold rise remain best.

Verification / Alternative check:
Many institutional algorithms in South Asia cite H ≥ 1:200 as significant when correlated with symptoms and timing; nevertheless, culture remains definitive.

Why Other Options Are Wrong:

• 1:50 and 1:100: often within background titres; low specificity.
• 1:400: highly specific but may miss true cases.
• Only fourfold rise: the most reliable, but the question asks for a single-sample significant titre in practice.

Common Pitfalls:
Over-reliance on a single Widal titre without culture confirmation; ignoring regional baseline data and timing relative to illness onset.

Final Answer:
1:200 or more.