Diphtheria Workup

Updated: Aug 17, 2022
  • Author: Bruce M Lo, MD, MBA, RDMS, FACEP, FAAEM, FACHE, FAAPL, CPE; Chief Editor: Jeter (Jay) Pritchard Taylor, III, MD  more...
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Laboratory Studies

To establish the diagnosis of C diphtheriae, it is vital to both isolate C diphtheriae in culture media and to identify the presence of toxin production. [3]

Bacteriologic testing

Gram stain shows club-shaped, nonencapsulated, nonmotile bacilli found in clusters.

Immunofluorescent staining of 4-hour cultures or methylene blue–stained specimen may sometimes allow for a speedy identification.


Inoculate tellurite or Loeffler media with swabs taken from the nose, pseudomembrane, tonsillar crypts, any ulcerations, or discolorations. Identification is accomplished through observation of colony morphology, microscopic appearance, and fermentation reactions. Any diphtheria bacilli isolated must be tested for toxin production.

Obtain throat and pharyngeal swabs from all close contacts. [11]


Toxigenicity testing is aimed at determining the presence of toxin production.

Elek test detects the development of an immunoprecipitin band on a filter paper impregnated with antitoxin and then is laid over an agar culture of the organism being tested. [14]

Polymerase chain reaction (PCR) assays for detection of DNA sequence encoding the A subunit of tox+ strain are both rapid and sensitive.

Once diphtheria infection has been established, the Centers for Disease Control and Prevention (CDC) should be contacted, and further testing may be requested.

Other laboratory studies

CBC may show moderate leukocytosis.

Urinalysis (UA) may demonstrate transient proteinuria.

Serum antibodies to diphtheria toxin prior to administration of antitoxin: Low levels cannot exclude the possibility of the disease; high levels may protect against severe illness (concentrations of 0.1 to 0.01 IU are thought to confer protection). [11]

Serum troponin I levels seem to correlate with the severity of myocarditis. [25]


Imaging Studies

Chest radiograph and soft tissue neck radiography/CT or ultrasonography may show prevertebral soft tissue swelling, enlarged epiglottis, and narrowing of the subglottic region.

Echocardiography may demonstrate valvular vegetations; however, this systemic manifestation of diphtheria is rare. [25, 26]


Other Tests

ECG may show ST-T wave changes, variable heart block, and dysrhythmia.



The following procedures may be necessary:

  • Endotracheal intubation
  • Surgical airway - Cricothyroidotomy or tracheostomy
  • Laryngoscopy, bronchoscopy as indicated in intubated patients
  • Electrical pacing for high-grade conduction disturbances