Serology in enteric fever: In the Widal agglutination test, what is generally considered a significant diagnostic titre for O (somatic) agglutinins in a single sample, assuming an endemic setting and appropriate clinical correlation?

Introduction / Context:
The Widal test measures agglutinating antibodies against Salmonella enterica serovars Typhi and Paratyphi antigens (O and H). While culture remains the gold standard, Widal is still used in resource-limited settings. Knowing typical significance thresholds helps avoid over- or under-diagnosis.

Given Data / Assumptions:

• Endemic area with baseline titres present in the population.
• Single-sample interpretation (rise in paired sera is better but often unavailable).
• We focus on O agglutinins, which tend to rise early and indicate active infection.

Concept / Approach:
In many endemic regions, an O agglutinin titre of 1:100 or greater is commonly taken as significant in the right clinical context, recognizing that local baseline titres and lab-specific cutoffs vary. Lower titres (e.g., 1:50) are often non-specific; higher cutoffs (e.g., 1:150) may be too stringent in some locales.

Step-by-Step Solution:
Identify standard threshold used in many labs for O antibodies.Balance sensitivity and specificity in endemic settings.Select 1:100 or more as a generally significant titre (with clinical correlation).Note that paired sera showing a fourfold rise is most diagnostic.

Verification / Alternative check:
Local public health guidelines often publish baseline titre distributions; many cite O ≥ 1:100 as significant when supported by symptoms and epidemiology.

Why Other Options Are Wrong:

• 1:50: too low in endemic regions; poor specificity.
• 1:150: may miss true positives where background levels are lower.
• 1:2.00: not a valid dilution expression (likely intended 1:200); overly stringent for many settings.

Common Pitfalls:
Relying solely on a single Widal titre without considering clinical picture, vaccination history, or alternative diagnoses.

Final Answer:
1:100 or more