Diagnosis of Tuberculosis: Part 1 - Demonstration of Causative Organism

Tuberculosis (TB), despite being a preventable and treatable infection, still remains a major cause of mortality worldwide. Delay in establishing the diagnosis signifi-cantly contributes to the mortality. Early diagnosis, especially in children, is faced with several challenges. This article reviews the current knowledge and challenges of TB diagnosis. The philosophy of TB diagnosis is based on two principles: (1) demonstration of the causative organism in the host, and (2) demonstration of the host-reaction to the invading pathogen. Staining and culture demonstrate intact bacilli, while molecular methods demonstrate cellular fragments of mycobacteria. Staining techniques use a unique property of mycobacterial cell wall that resists decolourization with acid-alcohol. Fluorescent stains (e.g. auramine-rhodamine) are better than the conven-tional Ziehl-Neelsen stain. When the bacterial load is too low to be detected in stained smears, culturing is used to increase the bacterial density. The WHO recom-mends Bactec MGIT-960™ medium because of its better yield than the conventional Lowenstein-Jensen medium. Dead or multiplying bacilli shed their genetic materials (DNA and RNA) as well as cell wall antigens into the host secretions or circulation. Techniques like polymerase chain reaction amplify the miniscule amount of bacterial nucleic acids and facilitate their detection. The specificity and sensitivity of these techniques are often greater than 90%. Hence, the WHO recommends cartridge-based nucleic acid amplification test (CB-NAAT) as the preferred investigation of childhood TB. Lipoarabinoman- nan, a cell wall glycolipid specific to mycobacteria, is a useful urinary biomarker of TB, especially in HIV patients.