Diphtheria laboratory findings

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Dima Nimri, M.D. [2]

A presumptive diagnosis of diphtheria is usually made based on an observation of clinical features. A definitive diagnosis is made by growing the specific Corynebacterium species on special cultures from the respiratory tract secretions or cutaneous lesions. Other tests include toxin detection via various methods.

A presumptive diagnosis of diphtheria is usually made based on clinical features. A definitive diagnosis can be made by growing the specific Corynebacterium species on special cultures from the respiratory tract secretions or cutaneous lesions. Other tests include toxin detection via various methods.^[1][2][3]

The diagnosis of diphtheria is made by culture and special staining for the different Corynebacterium species from respiratory tract secretions or cutaneous lesions:^[1][2][3][4]

• Gram stain: Gram positive rods in a distribution that is said to resemble Chinese characters
• Tellurite stain: Gray-black colonies
• Tindale: Black colonies with halos around it and an associated garlic odor
• Loffler medium: Metachromatic staining

Most species of Corynebacterium are catalase-positive, urea-negative, nitrate-positive, pyrazinamidase-negative and cystinase-positive.^[1]

The toxin for Corynebacterium diphtheria can be detected through a variety of methods, which include:^[1][2]

• Elek’s test
• PCR: a positive assay for the diphtheria toxin gene is not confirmatory, but rather suggests that Corynebacterium diphtheria is the causative agent. However, a negative assay does rule it out.
• Enzyme immunoassay (EIA)

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