A 4-year-old unvaccinated child from Delhi presents with a thick, leathery pseudomembrane covering the pharynx and extending into the larynx, with stridor and respiratory distress. Throat swab has been sent for culture. Which is the most appropriate confirmatory test to identify toxin-producing strains of *Corynebacterium diphtheriae* from the culture isolate?

## Confirmatory Test for Toxin-Producing C. diphtheriae **Key Point:** The **Elek immunodiffusion test** is the gold standard confirmatory test to detect diphtheria toxin production from isolated *C. diphtheriae* strains. It is more specific than biochemical tests because it directly detects the toxin protein. ### Diagnostic Hierarchy for C. diphtheriae ```mermaid flowchart TD A[Throat swab/Culture]:::outcome --> B[Grow on Löffler's serum medium]:::action B --> C{Gram-positive bacilli with<br/>metachromatic granules?}:::decision C -->|Yes| D[Biochemical tests:<br/>Catalase, Fermentation]:::action D --> E{Catalase +ve?<br/>Glucose + Maltose fermentation?}:::decision E -->|Yes| F[Presumptive C. diphtheriae]:::outcome F --> G[Elek immunodiffusion test]:::action G --> H{Toxin production<br/>detected?}:::decision H -->|Yes| I[Toxin-producing strain<br/>CONFIRMED]:::outcome H -->|No| J[Non-toxigenic strain]:::outcome ``` ### Elek Immunodiffusion Test: Mechanism | Feature | Detail | |---|---| | **Principle** | Antigen–antibody precipitation in agar; diphtheria antitoxin (antibody) diffuses from a strip embedded in agar and reacts with toxin produced by the bacterial isolate | | **Substrate** | Special agar plate with diphtheria antitoxin-impregnated strip (filter paper) placed centrally | | **Inoculation** | Isolated *C. diphtheriae* colonies streaked perpendicular to the antitoxin strip | | **Incubation** | 24–48 hrs at 35–37°C in CO~2~ | | **Positive result** | White precipitate line forms between bacterial growth and antitoxin strip (toxin–antitoxin complex) | | **Sensitivity** | ~95% for toxin-producing strains | | **Specificity** | ~99% (detects only functional toxin) | **High-Yield:** The Elek test is **specific for toxin production**, not just organism identification. A strain may be *C. diphtheriae* (confirmed by biochemistry) but non-toxigenic (Elek negative) — such strains do NOT cause diphtheria and do NOT require antitoxin therapy. **Clinical Pearl:** Respiratory diphtheria with laryngeal involvement (as in this case) is a medical emergency. Antitoxin therapy is initiated **before** culture results are available, based on clinical diagnosis. The Elek test is performed retrospectively for confirmation and epidemiological surveillance. ### Why Other Tests Are Insufficient | Test | Why Not Confirmatory for Toxin | |---|---| | **CAMP test** | Identifies *Streptococcus agalactiae*; not applicable to *C. diphtheriae* | | **Catalase test** | Confirms *C. diphtheriae* genus but does NOT detect toxin production | | **Fermentation tests** | Identify species/biotype but do NOT indicate toxigenicity | **Mnemonic:** **ELEK = Exotoxin detection by immunoLEgic (antigen–antibody) reaction** — remember that Elek specifically detects the **diphtheria exotoxin**, not just the organism. [cite:Park 26e Ch 24; Textbook of Microbiology (Ananthanarayan & Paniker) Ch 16]