## Diagnostic Timeline in Enteric Fever ### Sensitivity of Tests by Week of Illness | Test | Week 1 | Week 2 | Week 3 | Week 4+ | |------|--------|--------|--------|----------| | **Blood culture** | 90% | 75% | 30–40% | <10% | | **Stool culture** | 5–10% | 30% | 60–70% | 80–90% | | **Urine culture** | Rare | 5–10% | 20–30% | 40–50% | | **Widal test** | Low | Moderate | High | High | **Key Point:** Blood culture is gold standard in weeks 1–2 (highest sensitivity). By week 3, stool culture becomes the most reliable test, with sensitivity reaching 60–70%. ### Why Stool Culture in Week 3? 1. Bacteria localize in the Peyer's patches and gallbladder by the second week 2. Fecal shedding increases progressively through the second and third weeks 3. Blood culture sensitivity drops sharply after week 2 as bacteremia wanes 4. Stool culture remains positive for weeks even after clinical recovery (chronic carriers shed for months to years) **High-Yield:** The question specifies "third week" — this is the critical temporal clue. NEET PG loves testing the diagnostic algorithm across the illness timeline. **Clinical Pearl:** Widal test has poor specificity in endemic areas (India, Southeast Asia) due to background seropositivity from previous infections or vaccination; it is NOT a reliable diagnostic test and is increasingly discouraged in favor of culture and PCR. **Mnemonic:** **BSUW** — **B**lood early, **S**tool late, **U**rine uncommon, **W**idal weak (in endemic areas).
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