Diagnosis of diphtheria based exclusively upon direct microscopy of a smear is unreliable. Culture specimens should be initially plated on blood agar and Hoyles tellurite agar and incubated at 35 o C aerobically. Plates are incubated for 18 to 48 hours. Screen plates for typical dull black colonies by subculturing each morphotype to modified Tinsdale agar and blood agar. Incubate subcultured plates at 35 o C aerobically for 24 hours.
Any colonies showing brownish-black halos are positive.
Any gram-positive bacilli seen on gram stain that exhibit typical colonial morphology should be submitted to a reference facility for further identification and confirmation. All isolates identified as C. diphtheriae should be tested for toxigenicity at a reference laboratory. The current method used is the Elek test. Molecular methods for the determination of toxigenicity are being developed. Although most isolates are susceptible to penicillin, susceptibility testing may be undertaken at a reference laboratory.