## Diagnosis of Enteric Fever by Stage of Illness **Key Point:** The sensitivity of diagnostic investigations in enteric fever varies dramatically with the stage of illness. ### Timing of Culture Positivity | Investigation | Week 1 | Week 2–3 | Week 3+ | Sensitivity | |---|---|---|---|---| | **Blood culture** | 80–90% | 40–50% | <10% | Best in 1st week | | **Bone marrow culture** | 80–90% | 80–90% | 80–90% | Consistent across all stages | | **Stool culture** | <5% | 20–30% | 40–50% | Increases after week 2 | | **Urine culture** | <5% | 10–15% | 20–30% | Low sensitivity | **High-Yield:** Bone marrow culture is the gold standard investigation for enteric fever diagnosis, especially after the first week when blood culture sensitivity falls sharply. It remains positive even in the second and third weeks of illness. ### Clinical Pearl In this patient (day 10 of illness), blood culture sensitivity has already declined significantly. Bone marrow culture has superior sensitivity across all stages and is unaffected by prior antibiotic use. ### Why Bone Marrow Culture Is Superior 1. **Highest sensitivity** (80–90%) regardless of stage of illness 2. **Not affected by prior antibiotics** (unlike blood culture) 3. **Positive even in chronic carriers** 4. **Allows organism isolation** for culture and sensitivity testing **Mnemonic:** **BM-BEST** — Bone Marrow is Best for Enteric fever diagnosis in Stage Two and Third weeks. ### Diagnostic Algorithm ```mermaid flowchart TD A[Suspected Enteric Fever]:::outcome --> B{Stage of Illness?}:::decision B -->|Week 1| C[Blood Culture]:::action B -->|Week 2-3| D[Bone Marrow Culture]:::action B -->|Week 3+| E[Stool Culture + Bone Marrow]:::action C --> F[80-90% Sensitivity]:::outcome D --> G[80-90% Sensitivity]:::outcome E --> H[Best combination]:::outcome ``` [cite:Harrison 21e Ch 157]
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