What laboratory tests are essential for confirming a diagnosis of diphtheria in a suspected case?

Essential laboratory tests for confirming a diagnosis of diphtheria include:

• Culture of throat or lesion swabs on selective media to isolate Corynebacterium diphtheriae or C. ulcerans, which is the primary diagnostic step ¹.
• Microscopic examination of the specimen using Albert’s stain or Gram stain to identify characteristic gram-positive bacilli with metachromatic granules supports the diagnosis ¹.
• Biochemical identification and toxigenicity testing of the isolated strain are crucial to confirm the presence of toxigenic C. diphtheriae, as only toxigenic strains cause diphtheria ¹.
• Detection of diphtheria toxin production is essential and can be performed by the Elek immunodiffusion test, which remains the gold standard for confirming toxigenicity ¹.
• Molecular methods such as PCR targeting the tox gene can rapidly detect toxigenic strains and complement culture methods, especially when toxin expression is low or culture is negative (Efstratiou et al., 2000).
• Recent literature highlights challenges in toxin detection and suggests that combining culture, tox gene PCR, and Elek test improves diagnostic accuracy (Prygiel et al., 2024).

In summary, the essential laboratory confirmation of diphtheria requires isolation of the organism from clinical specimens, demonstration of toxigenicity by Elek test or molecular methods, and biochemical identification to differentiate toxigenic strains ¹ (Efstratiou et al., 2000; Prygiel et al., 2024).