Laboratory Diagnosis of Tuberculosis

Although sputum microscopy remains the most widely used diagnostic method worldwide, given its low sensitivity (50-80%) and specificity (it does not distinguish M. tuberculosis from nontuberculous mycobacteria), more rapid, sensitive, and specific methods are required nowadays. Culture for mycobacteria continues to be the gold standard due to its higher sensitivity and specificity, and because it also allows the detection of strains resistant to antituberculosis drugs; however, even with liquid culture media, at least one month of processing is required to obtain results. Therefore, rapid genotyping methods (e.g., Xpert and line-probe assays known as LPAs) have replaced phenotypic methods by allowing the identification of species and the presence of mutations associated with resistance in less than 24 hours. The Xpert, an automated real-time PCR system, can identify the presence of mutations associated with rifampin resistance in less than two hours, with a sensitivity higher than 70% in patients with negative microscopy. LPAs allow species identification and the presence of mutations associated with resistance to isoniazid, rifampin, fluoroquinolones, and second-line injectables in less than 24 hours. Progressively, the complete sequencing of the Mycobacterium tuberculosis genome has been integrated into the diagnostic protocol, allowing the identification of all mutations associated with resistance for all antituberculosis drugs. Phenotypic methods (microscopy and cultures) continue to play an essential role in the follow-up of patients who are already under treatment.

Keywords: Cultures, Diagnosis, Genome sequencing, LPA, MGIT, Microscopy, Tuberculosis, Xpert.Cultures, Diagnosis, Genome sequencing, LPA, MGIT, Microscopy, Tuberculosis, Xpert.