Rapid detection of mycobacteria
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
From 1985 to 1992, the number of reported cases of tuberculosis increased 18%. This infectious disease still kills an estimated 3 million persons a year worldwide, making it the leading infectious disease cause of death.(1) Between 1981 and 1987, AIDS has disseminated nontuberculous mycobacterial infections; eg, Mycobacterium intracellulare (MAC). By 1990, the increased number of disseminated nontuberculous mycobacterial infections resulted in a cumulative incidence of 7.6%.(2) In addition to the resurgence of Mycobacterium tuberculosis (MTB), multidrug-resistant MTB (MDR-TB) has become an increasing concern. Laboratory delays in the growth, identification, and reporting of these MDR-TB cases contributed, at least in part, to the spread of the disease.(3)
The US Centers for Disease Control and Prevention (CDC) have recommended that every effort must be made for laboratories to use the most rapid methods available for diagnostic mycobacteria testing. These recommendations include the use of both a liquid and a solid medium for mycobacterial culture, preferably using an automated system.(3,4)
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
A final negative report is issued after 60 days incubation.
Positive cultures are reported as soon as detected.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Recovery of mycobacteria in the BBL MGIT tube is dependent on the number of organisms present in the specimen, specimen collection methods, methods of processing, and patient factors such as presence of symptoms prior to treatment.
The use of BBL MGIT PANTA antibiotic mixture, although necessary for all nonsterile specimens, may have inhibitory effects on some mycobacteria.
For over 20 years the gold standard detection system for propagating mycobacteria in vitro has been the Bactec 460 instrument. The radiometric systems uses carbon(14) radiolabeled palmitic acid as a substrate, which when metabolized by an organism, produces detectable amounts of carbon-labeled carbon dioxide. The MGIT system uses fluorescence technology to detect microbial growth. The Bactec 460 and MGIT systems were compared. A total of 1,963 patient specimens, including 1,519 respiratory tract specimens that required decontamination with sodium hydroxide, and 444 sterile specimens that did not need to be decontaminated, were cultured. A total of 168 cultures grew acid-fast bacilli in 1 or both systems (8.5% positivity rate). The contamination rate for respiratory tract specimens positive in the Bactec 460 was 3.8% and 7.9% in the MGIT. Contamination of sterile specimens was 6.3% in the Bactec 460 and 10.1% in the MGIT. Combined rates were 4.3% for the Bactec 460 and 8.4% for the MGIT. The overall recovery rates for mycobacterial species, excluding Mycobacterium gordonae, were 82.8%, 79.1%, and 78.4% for the Bactec 460, MGIT 960, and solid media respectively. Recovery rates for the Bactec 460 and MGIT 960 were considered to be equivalent.
Clinical Reference Provides recommendations for further in-depth reading of a clinical nature
1. Bloom BR, Murrary CJ: Tuberculosis: commentary on a reemergent killer. Science 1992;257:1055-1064
2. Horsburgh CR, Jr: Mycobacterium avium complex infection in the acquired immunodeficiency syndrome. N Engl J Med 1991;423:1332-1338
3. Tenover FC, Crawford JT, Huebner RE, et al: The resurgence of tuberculosis: is your laboratory ready? J Clin Microbiol 1993;31:767-770
4. Cohn ML, Waggoner RF, McClatchy JK: The 7H11 medium for the cultivation of mycobacteria. Am Rev Resp Dis 1968;98:295-296
5. Youmans GP: Cultivation of mycobacteria, the morphology and metabolism of mycobacteria. In Tuberculosis. Edited by GP Youmans, Philadelphia, WB Saunders Company, 1979, pp 25-35
6. Kent PT, Kubica GP: Public health mycobacteriology: a guide for the level III laboratory. USDHHS, Centers for Disease Control, Atlanta, GA, 1985