Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1339 page)

   Respiratory or cutaneous specimens are inoculated onto SBA or CNA agar (to detect other pathogens), and agar-enriched media containing cystine and tellurite, such as cystine–tellurite blood agar or modified Tinsdale agar for
C
.
diphtheriae
detection.

   On cystine–tellurite agar,
C
.
diphtheriae
,
Corynebacterium ulcerans
, and
Corynebacterium pseudodiphtheriae
produce black colonies surrounded by a dark halo.

   The identification of suspect colonies must be confirmed. Toxin-producing and non–toxin-producing strains of
C
.
diphtheriae
may be isolated from clinical specimens.
C
.
diphtheriae
isolates should be referred for toxin production testing.

   
Turnaround time:
48–72 hours for initial isolation. Additional time is required for confirmation of the identity, toxin testing, and further characterization of suspicious isolates.

   Special Collection and Transport Instructions

   The laboratory should be alerted before specimen submission to ensure that appropriate medium is available for culture inoculation.

   Swab specimens are collected from multiple inflamed sites of the pharynx or other respiratory mucosal surfaces. Collection of specimens from near or under any diphtheritic membrane is recommended.

   Aspirate, swab, or tissue samples for detection of cutaneous diphtheria are collected following general collection recommendations for skin lesions.

   Routine transport media can be used for transport.

   Interpretation

   
Expected results:
No growth.

   
Positive results:
Isolation of toxigenic strains of
C
.
diphtheriae
from upper respiratory or cutaneous lesions is diagnostic of diphtheria.