A 6-year-old unvaccinated child from rural Maharashtra presents with a 3-day history of sore throat, low-grade fever, and difficulty swallowing. On examination, there is a thick, adherent, greyish-white membrane over the tonsils and pharynx that bleeds on attempted removal. The child also has cervical lymphadenopathy and mild stridor. Which investigation is most appropriate to confirm the diagnosis of diphtheria?

Explanation

## Diagnosis of Diphtheria — Investigation of Choice ### Gold Standard Investigation **Key Point:** Culture on Loeffler's medium (or tellurite medium) is the gold standard for confirming *Corynebacterium diphtheriae* infection. Gram stain showing Gram-positive bacilli in a characteristic "Chinese letter" or "cuneiform" arrangement supports the diagnosis. ### Why Loeffler's Medium? Loeffler's medium is a selective, enriched medium that: - Contains egg serum, which favours growth of *C. diphtheriae* - Allows visualization of metachromatic (Babes-Ernst) granules when stained with Albert's or Loeffler's methylene blue stain - Demonstrates the characteristic colonial morphology (small, greyish-white colonies) - Permits toxin production testing (Elek test for toxigenic strains) ### Specimen Collection **Clinical Pearl:** Swabs should be taken from: - The membrane itself (pseudomembrane) - The edge of the membrane - Nasopharynx (if nasal diphtheria is suspected) - Cultures should be obtained **before** or **early** in the course of antibiotics, as treatment rapidly sterilizes the throat ### Toxin Confirmation **High-Yield:** The Elek test (immunodiffusion test) confirms toxin production by the isolate — only toxigenic strains cause systemic complications (myocarditis, neuropathy). Non-toxigenic strains cause local infection only. ### Timing of Diagnosis **Warning:** Treatment with diphtheria antitoxin and antibiotics should **NOT** be delayed waiting for culture confirmation. Clinical diagnosis (membrane + systemic features) is sufficient to initiate therapy immediately. ### Alternative/Rapid Methods - **PCR** for *C. diphtheriae* (faster, increasingly available) - **MALDI-TOF mass spectrometry** (rapid organism identification from culture) - Gram stain alone is supportive but NOT diagnostic (many organisms appear similar) [cite:Park 26e Ch 9]