Culture allows diagnostic confirmation of TB and is more sensitive than microscopy, 10-100 bacilli/ml are required to obtain a positive result3. Only specialized laboratories with regular quality assurance procedures in place can be relied upon for culture.
After decontamination of the sputum specimen to eliminate other organisms, the sample is centrifuged. The sediment is cultured in a special medium, in an incubator at 37°C. For specimen storage and shipment, see Appendix 1.
M. tuberculosis is a slow-growing pathogen thus, culture results are obtained after several days. The turn around time (TAT) for these techniques is summarized in Section 3.5, Table 3.1.
Culture should play a bigger role in diagnosis and patient follow-up due to the limited value of direct microscopy for:
– Confirmation of failures;
– Diagnosis of EPTB;
– Confirmation of smear negative TB when the diagnosis is in doubt;
– Distinction between M. tuberculosis complex and NTM;
– Monitoring treatment and outcome evaluation for patients on second-line anti-TB drugs.
Once there is growth on either a solid or liquid media, the organism must be identified. There are a number of ways to identify M. tuberculosis. The tests can be phenotypic (the most common being the niacin test) or genotypic (which use DNA analysis, Section 3.4). Given the complexities associated with phenotypic identification, genetic tests are preferred. The drawback is their cost. Nonetheless, laboratories performing cultures, at a minimum, should be able to conduct identification tests for M. tuberculosis that follow international guidelines.