Clinical usefulness of detecting growth of Mycobacterium tuberculosis in positive Bactec phials using PCR

Direct detection of Mycobacterium tuberculosis in respiratory samples from patients in Scandinavia by polymerase chain reaction

OBJECTIVE: To investigate the use of DNA amplification by the polymerase chain reaction (PCR) for the detection of Mycobacterium tuberculosis directly in human respiratory specimens. METHODS: The PCR assay employed was the Amplicor M. tuberculosis Test (Roche Diagnostics, Switzerland), which uses the 16S rDNA as the target template. Nine hundred and sixty samples from 741 patients in two clinical microbiology laboratories in Norway and Sweden were processed by routine culture analysis and PCR. RESULTS: Of the 56 specimens containing cultivatable M. tuberculosis, 49 (87.5%) were detected by PCR. Among the 904 culture-negative specimens, 897 samples were negative also by PCR and seven (0.8%) were positive by PCR. In comparison with culture, the sensitivity, specificity, and positive and negative predictive values of PCR were 91.7%, 99.6%, 94.2% and 99.4% for laboratory 1 and 80.0%, 98.7%, 76.2% and 99.0% for laboratory 2, respectively. For both laboratories combined the values were 87.5%, 99.2%, 87.5% and 99.2%. CONCLUSIONS: These results indicate that multiple (two or three) respiratory samples from each patients should be tested, to allow sufficient accuracy in detecting M. tuberculosis in the specimens. Still, the labor-intensive format of this test necessitates strong clinical indications and patient prioritization to provide a service feasible within the current limits of routine laboratories.

Early detection of Mycobacterium tuberculosis in BACTEC cultures by ligase chain reaction

The LCx Mycobacterium tuberculosis ligase chain reaction system (Abbott Diagnostic Division, Abbott Park, Ill.) was used to detect M. tuberculosis in 150 consecutive BACTEC vials on the day on which a positive growth index (GI) was recorded. By LCx, M. tuberculosis DNA was detected in BACTEC vials on average 2.6 days before the presence of acid-fast bacilli could be confirmed by microscopic examination. A total of 106 of 108 M. tuberculosis isolates were detected without centrifugation from bottles presenting very low GIs (average, 70; median, 33). No false-positive result was obtained from nontuberculous mycobacteria or from isolates with contaminants.

Critical assessment of gene amplification approaches on the diagnosis of tuberculosis

The resurgence of tuberculosis prompted the development of a number of new options for the rapid laboratory diagnosis of Mycobacterium tuberculosis (MTB). One of the most promising and exciting methodologies has been the introduction of assays employing amplification technology to detect MTB directly in clinical specimens. Although amplification assays hold significant promise to improve the laboratory diagnosis of tuberculosis, the decision to perform or not perform these assays is complicated. The performance of in-house polymerase chain reaction (PCR) assays and two commercially-prepared assays. GenProbe's AMTD test and Roche's AMPLICOR PCR assay are reviewed. Regardless of the amplification format, all assays have decreased sensitivity with specimens that are acidfast bacilli (AFB) stain-negative. Data from these studies and others indicate possible potential pitfalls of amplification assays, those being sampling errors, the presence of substances in clinical specimens that inhibit the amplification assay, and clinical utility. In light of these findings, the possible roles for these assays in the clinical microbiology laboratory are reviewed. In addition, factors such as cost, assay performance, etc. are discussed in order to facilitate the decision-making process concerning whether an amplification assay would be appropriate in a particular laboratory setting.

Current techniques in mycobacterial detection and speciation

Tuberculosis, a health concern so well controlled in recent decades that eradication seemed imminent, is once again reaching epidemic proportions following the increasing prevalence of AIDS. One important means of curbing this resurgence is a robust method that has the capability of identifying and speciating mycobacterial infections in a matter of days. Classic biochemical techniques, which require 4 to 8 weeks to identify and speciate tuberculosis infections, are in the process of being replaced by newer methods, including BACTEC, gene probes, nucleic acid amplification, amplification of ribosomal RNA, high-performance liquid chromatography, and gas chromatography-mass spectrometry. This review is intended to give the reader a synopsis of the current literature and research on these methods, including reliability, approximate time required for detection and speciation, and clinical utility.

Ability of PCR assay to identify Mycobacterium tuberculosis in BACTEC 12B vials

Introduction of PCR to directly detect Mycobacterium tuberculosis in clinical specimens has shown promise; however, interfering substances in clinical material have contributed to lowered assay sensitivities. We evaluated the ability of a PCR assay to detect M. tuberculosis in BACTEC 12B broth cultures. Clinical specimens were processed and inoculated into BACTEC 12B vials. Evaluation was approached in two phases, starting with an initial evaluation in which an aliquot of 12B broth was removed when the growth index (GI) was > or = 10 and stored at 4 degrees C until assayed by PCR. Of the 290 specimens initially assayed, 129 were culture negative for mycobacteria as well as PCR negative for M. tuberculosis. Except for one, cultures (n = 102) which grew mycobacteria other than M. tuberculosis were all PCR negative. The remaining 59 broths were all culture and PCR positive for M. tuberculosis; 39% (n = 23) of these cultures when assayed by PCR had GIs of < or = 50. Following initial evaluation, 200 12B BACTEC vials with GIs of > or = 10 were assayed in a similar manner except that specimens were amplified twice weekly to determine PCR's impact on the length of time to identification of M. tuberculosis as compared with standard laboratory practices. Utilization of PCR resulted in a mean time to detection of M. tuberculosis of 14 days, compared with 29 days by using commercially available nucleic acid probes to identify M. tuberculosis complex from growth of BACTEC 12B subcultures on solid media. In light of an overall sensitivity and specificity of 100 and 99.7%, respectively, coupled with the ability to identify M. tuberculosis days or weeks before other methods can be applied, we conclude that PCR might prove to be a rapid alternative for identification of M. tuberculosis in culture and allow for earlier setup of susceptibility testing.