Clinical Laboratory - Microbiology

Return to the main menu

Microbiologic Testing


Mycobacteria typically produce an inflammatory response with granulomatous disease. The most common organism is M. tuberculosis, while the most frequent "atypical" mycobacteria are M. kansasii and M. avium-complex. M. avium-complex is typically seen in immunocompromised individuals, such as those with HIV.

Mycobacteria, with mycolic acids in the outer cell wall, form stable complexes with dyes having aromatic methane rings such as fuchsin and auramine, and these complexes are resistant to decolorization with acid alcohols and are termed acid fast. The identification of "acid fast bacilli" is an initial screening test in pulmonary specimens or in tissues.

Culture is the most sensitive method for detection of M. tuberculosis in clinical specimens, however, they grow slowly compared to bacteria and cultures may be held up to 8 weeks before reporting no growth.

Clinical Diagnosis of Tuberculosis

Patients suspected of having latent or active infection with M. tuberculosis can be assessed in several ways.

  • Tuberculin skin test can be applied (using purified protein derivative, or PPD) which will elicit a delayed type hypersensitivity reaction leading to induration and erythema within 1 to 3 days if positive. In general, a zone of induration of at least 10 mm at 48 to 72 hours is indicative of infection with MTB. Persons previously receiving a BCG vaccination (made from an attenuated strain of M. bovis) will have a positive tuberculin skin test.

  • A chest radiograph may be obtained to assess evidence for pulmonary disease suggesting active mycobacterial infection.

  • Sputum samples can be obtained to screen for acid fast bacilli and/or culture and sensitivity. Cultures may require 2 months to confirm presence of pathogenic mycobacteria. The presence of acid fast bacilli indicates the need for immediate therapy and patient isolation to prevent spread to others.

  • A blood test for suspected MTB infection consists of an interferon-gamma release assay (IGRA) that measures the interferon-gamma released from sensitized T cells after in vitro stimulation with antigens specific for M. tuberculosis. Though there may be some cross-reactivity with M. kansasii, the antigens are highly specific for MTB.

A decision to give antimicrobial therapy depends upon the clinical scenario, modified by laboratory results that may come weeks to months later, indicating the mycobacterial species and its possible antibiotic resistance pattern.

Previous topic Next topic