A diagnostic kit shows promise for distinguishing between tuberculosis (TB) and its infections from disease caused by related mycobacteria family, which mimic TB and other lung disease in symptoms but need different clinical treatments.

Researchers report in a multi-center study that differentiating MAC-related pulmonary disease (MAC-PD) from TB can be done in a few hours using an assay that identifies antibodies specific to MAC. The research was published in the American Journal of Respiratory and Critical Care Medicine in the first April issue.

Mycobacteria tuberculosis, the bacterium that causes TB, comes from a larger family of mycobacteria, certain strains of which cause lung disease. The most common pathogenic nontubercular mycobacteria are known together as mycobacterium avium complex, or MAC. Distinguishing MAC-PD from TB is difficult and can take 8 weeks or more.

Further, MAC bacteria are ubiquitous in the environment and a positive culture can simply indicate specimen contamination.

MAC is responsible for a growing proportion of pulmonary disease, but how much is unclear. Some point to an escalating epidemic over the past two decades.
 
Distinguishing between MAC and TB has largely relied on a suite of clinical signs and obtaining repeatedly positive sputum cultures—a process that can be unwieldy and unreliable. Though the initial diagnosis may be uncertain, patients whose sputum is positive for acid-fast bacilli are often immediately isolated and sometimes started on a regimen of anti-TB drugs.

Isolating non-TB patients and beginning inappropriate treatment regimens drains resources that could be used to treat infectious TB, and may be a burden and risk to the patient. MAC is not contagious and sometimes requires no treatment.

To test the efficacy of the immunoassay kit, the researchers acquired specimens from six centers between June 2003 and December 2005. Samples came from 70 patients with MAC-PD, 18 with MAC contamination, 36 with pulmonary TB, 45 with other lung disease, and 76 from healthy patients.

They found serum antibody levels to the MAC-specific antigen were higher in patients with MAC pulmonary disease than those with other respiratory diseases, including tuberculosis. The sensitivity and specificity of the serologic test were 84.3% and 100%, respectively. The test took hours versus the 4 to 8 weeks it takes to determine conventional culture results.

To be validated, the kit must perform well with different populations and in different locations, as MAC strains can vary from place to place.