Pediatric Diphtheria Workup

Updated: May 02, 2019
  • Author: Cem S Demirci, MD; Chief Editor: Russell W Steele, MD  more...
  • Print

Laboratory Studies

Diagnostic tests used to confirm infection combine isolation of C diphtheriae on cultures with toxigenicity testing.

  • Bacteriologic culturing is essential to confirm the diagnosis of diphtheria.

    • In all patients in whom diphtheria is suspected and in their close contacts, obtain specimens from the nose and throat (ie, nasopharyngeal and pharyngeal swab) for culture.

    • Obtain a clinical specimen for culture as soon as possible when diphtheria (at any location) is suspected, even if treatment with antibiotics has been initiated.

    • Obtain specimens from the membrane as well as from the nose and throat. If possible, swabs also should be taken from beneath the membrane.

    • Alert the laboratory to the suspicion of diphtheria because isolation of C diphtheriae requires special culture media containing tellurite. C diphtheriae may be grown on various selective media, including tellurite agar or specially enriched Loeffler, Hoyle, Mueller, or Tinsdale medium.

    • Isolation of C diphtheriae from close contacts may confirm the diagnosis, even if results of cultures on specimens taken from the patient are negative.

    • After C diphtheriae has been isolated, determine the biotype: gravis, mitis, or intermedius (substrain).

  • Toxigenicity testing is also performed.

    • Perform toxigenicity testing using the Elek test to determine if the C diphtheriae isolate produces toxin.

    • Toxigenicity tests are not readily available in many clinical microbiology laboratories; send isolates to a reference laboratory with personnel proficient in performing the tests. The state health department or Centers for Disease Control and Prevention (CDC) can provide information on laboratories that offer this test (few laboratory staffs have the capability to test antibody levels).

    • Measurement of the patient's serum antibodies to diphtheria toxin before administration of antitoxin may help assess the probability of the diagnosis of diphtheria.

    • If antibody levels are low, diphtheria cannot be excluded, but if levels are high, C diphtheriae is less likely to produce serious illness.


Other Tests

Although no other tests for diagnosing diphtheria are commercially available, the CDC can perform a polymerase chain reaction (PCR) test on clinical specimens to confirm infection with a toxigenic strain.

  • The PCR test can detect nonviable C diphtheriae organisms from specimens taken after antibiotic therapy has been initiated.

  • Contact the state health department to report a suspected case and to arrange laboratory testing.

  • Although PCR results for the diphtheria toxin, as performed by the CDC diphtheria laboratory, provide supportive evidence for the diagnosis, data are not yet sufficient for PCR results to be accepted as a criterion for laboratory confirmation.

  • At present, a diagnosis of diphtheria should be classified as probable in a patient with positive results to PCR testing but in whom the organism was not isolated, histopathologic diagnosis has not been made, and no epidemiologic link can be made to a patient with laboratory-confirmed diphtheria.

  • When collecting specimens for culture, obtain additional clinical specimens for PCR testing at the CDC. Because isolation of C diphtheriae is not always possible (many patients have already received antibiotics for several days by the time a diphtheria diagnosis is considered), the PCR test can provide additional supportive evidence for the diagnosis of diphtheria.

  • The PCR assay allows detection of the diphtheria toxin gene (TOX).

  • Clinical samples (swabs, pieces of membrane, biopsy tissue) can be transported to the CDC with cold packs in a sterile empty container or in silica gel sachets. For detailed information on specimen collection and shipping and to arrange PCR testing, the state health department may contact the CDC diphtheria laboratory at (404) 639-1730 or (404) 639-4057.

  • Send all isolates of C diphtheriae, from any body site (respiratory or cutaneous), whether toxigenic or nontoxigenic, to the CDC diphtheria laboratory for reference testing. Clinical specimens should be sent to the CDC diphtheria laboratory for PCR testing. To arrange for specimen shipment, contact the state health department.