Diphtheria is usually diagnosed based on a patient’s clinical presentation. Presumptive treatment is then started while further tests to confirm the diagnosis are performed. The criteria for diagnosing this infection are outlined by the Centers for Disease Control and Prevention (CDC) and include both clinical and laboratory criteria.

The steps taken to diagnose diphtheria are outlined below:
 

In suspected cases of diphtheria it is important to start therapy as soon as possible, so presumptive therapy is usually started while further tests are arranged.

The diagnosis can be confirmed by culturing a swab sample from infected areas. A swab is taken from the throat, and in particular from the tonsillar crypts as well as any discolored or ulcerated areas.

The infected sample material is incubated in a nutrition medium. The preferred medium for diphtheria is one that contains tellurite, which gives the organism a selective growth advantage. A blood agar plate may also be used to check for the presence of other bacteria such as hemo¬lytic streptococcus.

If diphtheria is detected, the bacilli are checked to see if they are producing toxins.

A sample of the grey-white membrane that forms in the throat can be subjected to Gram staining in which gram-positive bacteria retain the dye after being washed with alcohol. On Gram staining, diphtheria bacilli appear as rod-shaped cells in a beaded arrangement.

To detect the infection among carriers or non-symptomatic diphtheria cases, material from a throat swab can be cultured using Löffler or Pai slant. After 18 to 24 hours of incubation, a medium containing tellurite is inoculated with growth from the slant.

CDC diagnosis criteria

According to the CDC, the following clinical and laboratory criteria need to be met for a diagnosis of diphtheria to be confirmed:

Clinical criteria

• Presence of upper respiratory tract infection along with sore throat
• Presence of high fever
• Presence of the grey white membrane or pseudomembrane over the throat, back of the mouth or tonsils

Laboratory criteria

• Presence of corynebacterium diphtheriae in the throat swab samples
• Histological or cellular level presence of the bacteria