Mycobacterium avium Complex Susceptibility−Broth Dilution

Mycobacterium avium complex isolated from a primary clinical specimen, on a submitted AFB conventional solid medium, or an AFB broth medium

Pure culture isolate on an AFB conventional solid medium or a minimum of 1 mL of AFB broth medium

Conventional or broth medium, tightly sealed, in etiologic agent packaging

Determine the susceptibility of Mycobacterium avium complex isolates to a profile of antimycobacterial agents. Testing is recommended for isolates from patients with previous macrolide therapy, patients who relapse while on macrolide therapy, or patients with initial isolates from blood, tissue, or clinically relevant respiratory specimens. Repeat susceptibility is recommended after 3 months (disseminated disease) or 6 months (chronic pulmonary disease) if the cultures are still positive or if there is no clinical improvement.

First line drugs for M. avium complex treatment includes clarithromycin and amikacin (IV or inhaled). Clarithromycin is the most appropriate class drug for the macrolides due to technical challenges with azithromycin. Second line drugs include moxifloxacin and linezolid, however, the in vivo effectiveness of these drugs for treatment of M. avium complex is unproven. These are the only drugs with CLSI interpretations for M. avium complex. Note that while ethambutol can be used as part of the treatment regimen for M. avium complex, studies have shown that MIC values do not correlate with clinical outcomes. For this reason, ethambutol is not reported for M. avium complex isolates.

Susceptibilities cannot be reported if the organism fails to grow in the test medium. Susceptibilities cannot be performed on mixed cultures.

This test was developed and its performance characteristics determined by Labcorp. It has not been cleared or approved by the Food and Drug Administration.

Sensititre® broth microdilution (MIC)

Failure to take all drugs in a multidrug regimen can lead to a shift toward resistant organisms and treatment failure. Nontuberculous mycobacteria, particularly strains of the M avium complex, are resistant to those drugs used for therapy of M tuberculosis.² Clarithromycin (and azithromycin) are the only agents for which CLSI interpretive guidelines are established. Treatment of MAC for most patients with nodular/bronchiectatic disease includes a three-times-weekly regiment of clarithromycin or azithromycin, rifampin, and ethambutol. For patients with MAC lung disease or severe nodular/bronchiectatic disease, a daily regime of clarithromycin or azithromycin, rifampin or rifabutin, and ethambutol with consideration of three-times-weekly amikacin or streptomycin early in therapy is recommended. Treatment should occur until the patient is culture negative for one year.³ Treatment of disseminated MAC disease should include clarithromycin or azithromycin and ethambutol with or without rifabutin until the symptoms have resolved and cell-mediated immune function has returned.