Susceptibility testing of Mycobacterium avium complex in clinical laboratories. Results of a questionnaire and proficiency test performance by participants in the College of American Pathologists Mycobacteriology E Survey.

OBJECTIVES

To obtain information regarding the frequency and methodology of susceptibility testing of Mycobacterium avium complex (MAC) in clinical microbiology laboratories, and to assess interlaboratory reproducibility of MAC susceptibility testing.

DESIGN

Questions addressing MAC susceptibility testing were added to the College of American Pathologists' 1994 Mycobacteriology E Proficiency Testing Survey, and participants were asked to complete the questionnaire. In addition, participants in the 1994 E Survey were asked to test susceptibility of a MAC isolate recovered from a proficiency testing specimen as an ungraded exercise if they offered such testing for patients.

RESULTS

Of the 1003 participants enrolled in the 1994 Mycobacteriology E-A Survey, 806 responded to one or more supplemental questions. In regard to the demand for MAC susceptibility testing, 606 participants indicated that the test is requested by physicians in their institutions, and 188 said that they do the test routinely on at least one MAC isolate per patient. Eighty-two percent (630/765) of participants refer the test to an outside laboratory, most commonly a commercial reference laboratory or state health laboratory. Of the 70 participants who perform MAC susceptibility testing in-house and indicated the method on the questionnaire, 54 (77%) used a solid medium, whereas only 14 (20%) used BACTEC TB, which currently is the recommended method. The most frequently tested drugs were ethambutol, rifampin, isoniazid, and streptomycin; other commonly evaluated agents were ciprofloxacin, amikacin, and clarithromycin. Only eight participants modify the pH of the medium when testing a macrolide. In regard to reporting test results, 56% (45/80) report a qualitative result only, 35% (28/80) report a quantitative result with a qualitative interpretation, and 9% (7/80) report only a quantitative result. Participant performance on the MAC proficiency testing specimen showed lack of interlaboratory reproducibility; 80% or fewer participants reported the correct result for all drugs except amikacin, for which 92% (11/17) of laboratories responded correctly.

CONCLUSIONS

Given the obvious interest in MAC susceptibility testing, standardized methodology that demonstrates interlaboratory reproducibility and, optimally, shows some correlation with clinical outcome is needed. Moreover, recommendations concerning indications for performing the test would be useful.