Diphtheria laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Kalsang Dolma, M.B.B.S.[2]

Laboratory Findings

A presumptive diagnosis is usually based on clinical features. Diagnosis is confirmed by isolating C. diphtheriae from culture of nasal or throat swabs or membrane tissue.

Culture and Staining

Diagnosis of diphtheria is usually made on the basis of clinical presentation since it is imperative to begin presumptive therapy quickly. Culture of the lesion is done to confirm the diagnosis. It is critical to take a swab of the pharyngeal area, especially any discolored areas, ulcerations, and tonsillar crypts. Culture medium containing tellurite is preferred because it provides a selective advantage for the growth of this organism. A blood agar plate is also inoculated for detection of hemolytic streptococcus. If diphtheria bacilli are isolated, they must be tested for toxin production. Gram stain and Kenyon stain of material from the membrane itself can be helpful when trying to confirm the clinical diagnosis. The Gram stain may show multiple club-shaped forms that look like Chinese characters. Other Corynebacterium species (diphtheroids) that can normally inhabit the throat may confuse the interpretation of direct stain. However, treatment should be started if clinical diphtheria is suggested, even in the absence of a diagnostic Gram stain.

Elek's test

It's worth noting that a low concentration of iron is required in the medium for toxin production; as at high iron concentrations, iron molecules bind to a repressor which shuts down toxin production^[1]. This is most appreciated when performing Elek's test for toxogenecity, in order to know if the organism is able to produce the diphtheria toxin or not.

PCR

PCR assays can also be performed on isolates, swabs, or membrane specimens to rapidly confirm the presence of the tox gene responsible for production of diphtheria toxin, but the test is available only in research or reference laboratories.

Gallery

Blood agar culture is growing colonies of Corynebacterium diphtheria var. belfanti bacteria. From Public Health Image Library (PHIL). ^[2]
Blood agar plate culture of Corynebacterium diphtheriae (gravis), smooth. From Public Health Image Library (PHIL). ^[2]
Blood agar plate culture of Corynebacterium diphtheriae mitis. From Public Health Image Library (PHIL). ^[2]
Cystine tellurite plate culture of Corynebacterium diphtheriae, displaying the gravis biotype colonial growth pattern. From Public Health Image Library (PHIL). ^[2]
McLeod's agar plate culture of Corynebacterium diphtheriae, gravis biotype. From Public Health Image Library (PHIL). ^[2]