Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1399 page)

   Interpretation

   
Expected results:
No growth.

   
Positive:
Growth of
M
.
tuberculosis
in culture is usually very specific for mycobacterial infection. Because of their wide distribution in the environment, cultures positive for NMTB species must be interpreted carefully, taking into consideration factors such as the species, number of positive cultures, and the patient’s clinical presentation.
M
.
gordonae
(tap water bacillus) is often isolated from patient specimens, but is rarely associated with disease; its growth is most likely caused by contamination of the specimen, or transient contamination of the patient, with organisms from external water sources.

   
Negative:
The post-test probability of mycobacterial infection is significantly diminished if cultures are negative, but additional cultures and specimens collected from different patient sites may be needed in patients with continued suspicion for mycobacterial disease in spite of initial negative cultures.

   Limitations

   Three or more specimens, and specimens from different sites, may be required for sensitive detection; invasive collection techniques may be needed. The final results of testing may not be available for up to 2 months after collection; decisions regarding empirical therapy and management may be required before culture results are available.

   Other Considerations

   The following mycobacterial species are most commonly associated with human disease:

   
M
.
tuberculosis
complex:
M
.
tuberculosis
,
M
.
africanum
(rare),
M
.
bovis
, including BCG, and
M
.
microti
(rare)—pulmonary and other localized infections and systemic disease

   
M
.
avium
complex (MAC)—systemic infection in immunocompromised patients, like patients with AIDS or chronic pulmonary disease

   
M
.
kansasii
—pulmonary disease

   Rapid growers:
M
.
fortuitum
,
M
.
chelonae
,
M
.
abscessus
—wound infections, localized and systemic infection