Accurate bacteriological diagnosis prior to starting anti-TB drugs is the best clinical and public health approach to TB: this is based on microscopic examination, solid or liquid culture and rapid or conventional DST. The latter is required to optimise the antibiotic combination, taking into account the resistance pattern of the isolated mycobacteria.
The turnaround time for direct microscopy of a sputum smear is 1 day; however, its diagnostic sensitivity is affected by the concentration of mycobacteria in the sample. This can be improved by centrifugation and by collection of at least two sputum specimens on different days (particularly early in the morning).
Due to the suboptimal sensitivity and specificity of microscopic examination, definite diagnosis requires trying to grow mycobacteria in culture. The mean time for detection of Mycobacterium complex is 3 weeks. New diagnostic methods such as nucleic acid amplification tests can reduce the time to presumptive bacteriological diagnosis, thereby increasing the pre-culture probability. Recently, similar techniques have been used for the rapid detection of resistance to MDR-defining drugs. The GeneXpert (Cepheid), for example, a new molecular technique, based on a nucleic acid amplification test, can detect whether TB and resistance to rifampicin (considered a reliable marker of MDR-TB) are present in less than 2 hours. It was recently endorsed by WHO because of its higher sensitivity (it detects from ~70% to > 90% of cases) and specificity (> 90% of uninfected patients will have a negative test); however, at present, cost limits the availability of this innovative technique in low-income countries.
Chest radiography and computed tomography are useful tools to complement bacteriological examinations in the diagnosis of TB. Radiography is commonly used to screen individuals with a significantly higher risk of TB than that of the general population (such as prisoners or contacts of infectious cases) and individuals with symptoms suggestive of TB.
Immunological tests, such as the tuberculin skin test and the recently introduced interferon-gamma release assays, are helpful for the diagnosis of latent TB but cannot replace bacteriological diagnosis of active TB.
The aims of anti-TB therapy are to cure the patient and to avoid the transmission of mycobacteria to other people. Treatment is characterised by an intensive phase (2 months) and a continuation phase (4 months). For new cases, the intensive phase usually includes four drugs (isoniazid, rifampicin, ethambutol and pyrazinamide) and is designed to eliminate actively growing, as well as semi-dormant, mycobacteria. The continuation phase, based on the combination of isoniazid and rifampicin, kills residual mycobacteria.
MDR- and XDR-TB require the use of second-line drugs, which are more expensive, toxic and difficult to manage. Treatment is prescribed for at least 20 months (with an intensive phase of at least 8 months) and should rely on at least four effective drugs.