## Investigation of Choice for Haemophilus influenzae ### Clinical Context Epiglottitis in a child is a medical emergency classically caused by *Haemophilus influenzae* type b (Hib), especially in unvaccinated populations. Rapid organism identification is critical for targeted antibiotic therapy. ### Why Chocolate Agar with X and V Factor Testing? **Key Point:** *Haemophilus influenzae* is a fastidious gram-negative coccobacillus with an absolute requirement for **X factor (haemin)** and **V factor (NAD)** for growth. These factors are present in chocolate agar (lysed RBCs), making it the gold standard culture medium. **High-Yield:** The organism does NOT grow on: - Plain nutrient agar (lacks X and V factors) - MacConkey agar (selective for enteric gram-negatives; *H. influenzae* is inhibited) - Blood agar (X factor present, but V factor is not bioavailable) **Clinical Pearl:** Chocolate agar culture combined with **X and V factor requirement testing** (using factor-impregnated discs or strips) provides definitive identification within 24–48 hours and is the standard confirmatory test in clinical microbiology laboratories. ### Supporting Investigations | Investigation | Role | Timing | |---|---|---| | Gram stain of blood | Presumptive (gram-negative coccobacillus) | Immediate, non-specific | | Chocolate agar culture + X/V factors | **Gold standard confirmatory test** | 24–48 hours | | PCR (ftsI gene) | Detects β-lactamase; not primary ID | Specialized labs only | | Latex agglutination (CSF) | For meningitis; not relevant in epiglottitis | Wrong specimen type | **Mnemonic:** **CHOC** = **C**hocolate agar for *H. influenzae* **H**aemin and **O**rganism **C**onfirmation. [cite:Mackie & McCartney Practical Medical Microbiology Ch 14]
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