BACTEC System May Eliminate Some Culture Needs
ABSTRACT & COMMENTARY
Synopsis: Use of the BACTEC system for mycobacterial cultures may eliminate the need to obtain three sputums on consecutive days in patients with suspected tuberculosis. One or two specimens may be sufficient, especially in patients who are smear-positive on the first specimen.
Source: Stone B, et al. The diagnostic yield of acid-fast bacillus smear-positive sputum specimens. J Clin Micro 1997;35: 1030-1031.
Stone and colleagues examined more than 11,500 specimens submitted for acid-fast smear and BACTEC culture to the Denver Health Medical Center Mycobacteriology Laboratory, which provides laboratory services for the Public Health Department and the county hospital and clinics. Two-thirds were sputum specimens. Of the 439 smear-positive sputum specimens, 387 (88%) grew mycobacteria. Of the 52 smear-positive specimens that failed to grow, 49 were obtained from patients with active TB receiving antituberculous therapy, and three were obtained from patients with a history of inactive TB. For smear-positive specimens, the authors concluded that the frequency of "falsely positive" cultures was, at most, 0.7%. Based on these data, Stone et al have altered their institutional policy such that only one additional sputum specimen is obtained for culture in patients whose first smear is positive.
COMMENT BY CAROL A. KEMPER, MD
Advances in both staining and culture techniques have increased the yield of mycobacterial cultures, raising questions regarding the necessity of collecting three sputums on consecutive days for diagnosing tuberculosis, especially in patients known to be smear-positive on the first specimen. Although these data suggest that a single specimen in someone known to be smear-positive is sufficient, the authors recommend obtaining a second specimen to ensure recovery in culture and enhance the identification of potentially troublesome organisms. The potential cost savings from this measure is small; only 124 sputum specimens, or 1% of the total number of specimens processed by the laboratory during the study period, would have been cancelled.
While 291 of 387 (75.2%) mycobacteria recovered from the culture-positive specimens were M. tuberculosis, organisms of the Mycobacterium avium complex (MAC) were found in 58 (15%) specimens. Other mycobacteria isolated from the culutre-positive specimens included M. kansasii (5.2%), M. xenopi (2.1%), M. genavense (1.6%), and one isolate each of M. gordonae, M. terrae, and a rapid grower (0.3%).
This rate of recovery of MAC is remarkably high, and it would be interesting to have more information on the patient population. Other institutions have also seen remarkable increases in the frequency of recovery of MAC from respiratory specimens.1-3 Prior to the AIDS "epidemic," the frequency of recovery of MAC from respiratory specimens at the San Francisco General Hospital was less than 0.5% in 1977; in 1984, this number had climbed to 6.5% and in 1988, to 8.8%.4 This increase is, in part, due to the impact of HIV infection but is evident at some institutions even when cases of AIDSare excluded. This phenomena, if true, suggests a shift in the epidemiology of pulmonary MAC, with greater numbers of infections occurring even in non-HIV infected patients.
References
1. Debrunner M, et al. Clin Infect Dis 1992;15:330-345.
2. Contreras MA, et al. Am Rev Respir Dis 1988;137: 149-152.
3. Du Moulin G, et al. J Clin Microbiol 1985;22:9-12.
4. Nassos PS, et al. Am Rev Respir Dis 1991;143:66-68.
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