A 6-year-old unvaccinated child from rural India presents with a thick, grey-white pseudomembrane over the tonsils and pharynx, fever, and cervical lymphadenopathy. Throat culture on selective media grows gram-positive bacilli. Regarding the laboratory diagnosis and management of diphtheria, all of the following are correct EXCEPT:

Question

Accepted Answer

B. The Schick test (intradermal toxin injection) is the gold standard for confirming active diphtheria toxin production in clinical cases. ## Laboratory Diagnosis of Diphtheria

**Key Point:** The Schick test is an **in vivo skin test** for immunity (antitoxin status), NOT a diagnostic test for active toxin production. It is now obsolete and rarely used clinically.

### Diagnostic Methods for C. diphtheriae

| Test | Purpose | Principle | Status |
|------|---------|-----------|--------|
| **Loeffler's Medium** | Primary isolation | Serum-enriched; enhances metachromatic granules | Gold standard for culture |
| **Elek Test** | Toxin detection | Immunodiffusion; toxin diffuses toward antitoxin-containing strip | Confirmatory; slow (24–48 h) |
| **PCR (*tox* gene)** | Rapid toxigenicity | Detects DNA of β-phage carrying *tox* | Fast, sensitive; now preferred |
| **Schick Test** | Immunity assessment | Intradermal toxin; blanching = immunity (antitoxin present) | **Diagnostic tool for immunity, NOT disease** |

### Why Option 1 Is Wrong

The **Schick test is NOT used to diagnose active diphtheria**. It is an **in vivo test of immunity** — it determines whether a person has circulating antitoxin (protective antibodies). A positive Schick (no blanching) indicates lack of immunity; a negative Schick (blanching) indicates immunity.

In a symptomatic patient with suspected diphtheria, diagnosis relies on:
1. **Culture** on Loeffler's medium
2. **Elek test** or **PCR** to confirm toxigenicity

**High-Yield:** This is a classic NEET PG trap. Students confuse the Schick test (immunity) with diagnostic tests (culture, Elek, PCR). The Schick test is now **obsolete** in clinical practice.

### Clinical Pearl

**Clinical Pearl:** In this case, the child requires **immediate antitoxin therapy** (diphtheria antitoxin, horse serum) before culture results are available, based on clinical suspicion alone. Antibiotics (benzylpenicillin or erythromycin) are adjunctive. Vaccination with diphtheria toxoid is given during convalescence.

### Mnemonic

**"ELPS for Diagnosis, Schick for Immunity"**
- **E** = Elek test (toxin detection)
- **L** = Loeffler's medium (culture)
- **P** = PCR (*tox* gene)
- **S** = Schick test (immunity assessment — NOT diagnosis)

[cite:Jawetz & Melnick 27e Ch 18; Park 26e Ch 3]

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Answer

A. Elek immunodiffusion test detects diphtheria toxin in culture filtrate and confirms toxigenicity

Answer

C. Loeffler's medium (serum-containing) is preferred for primary isolation and enhances metachromatic granule visibility

Answer

D. PCR targeting the *tox* gene provides rapid, sensitive detection of toxigenic C. diphtheriae

Explanation

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