Tests
1st tests to order
bacteriologic culture, microscopy and sensitivities
Test
Requires blood agar and a blood agar medium containing tellurite (Hoyle) or Tinsdale media.[41] Advance preparation and liaison with a microbiologist is required.[39][43]
Cultures should be obtained from nose and throat swabs in patients with suspected diphtheria and their close contacts. If possible, swabs also should be taken from beneath the pseudomembrane.
Cultures may be taken even if antibiotic treatment has been started, although culture is less likely to isolate the organisms in this circumstance. Isolated organisms should be tested for antimicrobial sensitivities.[44]
If transport of clinical samples cannot be done within 8 hours of collection, they should be stored at 4-8ºC and Amies transportation media (or a suitable alternative) should be used.[39]
Result
gray and waxy colonies on blood agar; black and glossy colonies on Hoyle media; black and brown colonies with halos on Tinsdale media, irregularly staining pleomorphic bacilli on microscopy.
Tests to consider
Elek test for toxigenicity
Test
A technically difficult test and unavailable in many laboratories, but the gold standard assay for phenotypic discrimination between toxigenic and nontoxigenic strains of C diphtheriae.[39][41] In the US, the Centers for Disease Control and Prevention (CDC) has the only laboratory able to perform Elek testing.[12]
Result
positive
polymerase chain reaction (PCR)
Test
PCR testing can provide supporting evidence for the diagnosis of diphtheria and help to discriminate between toxigenic and nontoxigenic strains by detecting genes associated with the diphtheria toxin (dtxR and tox).[39][40][41]
It discriminates toxigenic from nontoxigenic bacteria in most cases, though it is important to remember that nontoxigenic toxin gene bearing strains of C diphtheriae occasionally occur.[41] As culture-based isolation of C diphtheriae is often unsuccessful once antibiotic treatment has been initiated, PCR can be useful because it still identifies non-viable C diphtheriae organisms from clinical specimens.
Result
positive
diphtheria antibodies
Use of this content is subject to our disclaimer