The MTB are the small red rod shaped organisms. Auramine fluorescence stains are a newer adaptation of this test. It requires fluorescence microscopy but is a little more sensitive as the fluorescence is easier to see than the coloured stain.
Culture techniques are still seen as the gold standard for active TB as they are extremely sensitive, so long as live mycobacteria can be obtained in the sample. It is therefore the "gold standard" test which is used for comparison with other methods when calculating sensitivity in active disease. M. tuberculosis can be cultured from a variety of specimens including sputum, Central Spinal Fluid (CSF), pleural effusion, bronchoalveolar lavage (BAL), gastric aspirate etc and can thus be used to detect pulmonary as well as
non-pulmonary disease. By assessing the effect of antibiotics on the cultured bacilli, this technique can also identify the antibiotic susceptibility of the particular strain of TB infecting the patient. It is therefore the main method for identifying if a person has multi-drug resistant (MDR) TB. However, it is not always possible to obtain mycobacteria in the sample, especially in non-pulmonary TB so culture is not a sensitive test. If performed correctly it should have very high specificity and can distinguish M. tuberculosis from other mycobacteria. A drawback of this test is the time to result which can be anything from 2 to 6 weeks.
Nucleic acid amplification tests (NAATs), such as polymerase chain reaction (PCR), are a relatively new development in active TB testing. Even though NAAT techniques can magnify even the smallest amounts of genetic material, the sample used still has to contain a certain number of TB bacilli and this is not always possible, particularly with non-pulmonary TB where sensitivity can be as low as 60%. To increase the number of bacilli in the sample, and hence improve the sensitivity of the test, the laboratory will often culture the sample, to allow the bacilli to multiply, before carrying out the PCR test. This can take several
days or weeks. The main use of NAAT is not to diagnose TB per se, but to rule out infections caused by atypical mycobacteria in a sputum smear positive patient, before culture results are known. This helps treatment to be initiated quickly, with the therapy then being tailored to the patient based on the culture results obtained 6weeks later.
More recently NAATs have been used to identify MDR TB as mutations in the DNA of MTB which confer drug resistance have been discovered. These methods are quicker than culture but generally only identify resistance to rifampicin and isoniazid.