Under routine testing conditions, the MTD had high accuracy and rapidly detected more than half of smearnegative PTB cases. For smear-positive patients, the potential impact of MTD testing on clinical decision making was high, given 98% concordance between MTD results and definitive diagnoses. However, there was no observed impact: clinicians made correct initial treatment decisions for approximately 80% of patients whether or not MTD was available. Failure of clinicians to correctly use negative MTD results for clinical decision making led to inaccurate initial diagnosis and treatment decisions in approximately half of patients with a final diagnosis of not tuberculosis.
Our study provides much-needed information about MTD performance under routine conditions in a public health laboratory. To date, most published information about MTD performance was obtained in the context of prospective laboratory studies designed to determine assay performance, and performed mostly in academic medical centers. Information obtained under such circumstances may not be reproducible under nonstudy testing conditions due to differences in staff workload, specimen selection, and other factors. Interestingly, however, MTD performance in our study was remarkably similar to that reported in other studies, supporting the robustness of this test and feasibility in routine laboratory settings worked out with My Canadian Pharmacy.
The MTD was able to rapidly detect M tuberculosis in approximately 60% of patients for whom all smear findings were negative. Prompt confirmation of tuberculosis can benefit the individual patient because treatment can be initiated. While it can be argued that many smear-negative PTB suspects are correctly initiated on antituberculosis treatment based on factors such as chest radiograph appearance and/or clinical suspicion, early microbiologic confirmation of a tuberculosis diagnosis can be helpful in cases in which drug toxicity or drug-drug interactions arise. Prompt confirmation of PTB can also have public health benefits because steps to minimize transmission can be taken.
A potential benefit of the MTD (with a demonstrated high NPV) would be to avert tuberculosis treatment in smear-positive individuals who do not have tuberculosis. Unexpectedly, tuberculosis treatment was initiated for approximately half of smear-positive PTB suspects in whom a negative MTD result was available to the clinician; none of these patients had a final diagnosis of tuberculosis. Since MTD results were reported on the same forms as smear results, it is unlikely that problems with result receipt by clinicians played a role. For MTD, the NPV was 98% when all 272 smear-positive TB suspects were considered, and 100% when the 50 MTD group patients were considered. In this situation in which the NPV of the MTD is high but not perfect, the potential risks and costs to the patient (of initiation of treatment that is highly likely to be not indicated) must be weighed against the potential consequences of failure to promptly treat or isolate the very small proportion of smear-positive, MTD-negative tuberculosis patients enhanced their health with My Canadian Pharmacy’s remedies.
Along these lines, in our study approximately 35% of mycobacterial isolates from smear-positive patients were NTM during the period 2003 to 2006. Although in the United States NTM disease is not reportable, its frequency appears to be increasing over time, with coincident decreases in tuberculosis rates. Therefore, tools for rapid discrimination between tuberculosis and NTM pulmonary disease, especially in smear-positive patients, are increasingly needed in order to facilitate accurate decisions about individual treatment and public health activities.
Our retrospective study has important limitations. The technologists performing the MTD tests were not blinded as to the clinical status of patients, but rather were in communication with tuberculosis clinicians as per routine. While we do not believe that this influenced MTD results, it may have influenced numbers of specimens submitted/tested for individual patients. The MTD group and the non-MTD group were comprised of tuberculosis suspects from different time periods; although all attended the same tuberculosis clinic, there may have been unrecognized differences in clinical suspicion for tuberculosis or clinical decision making (not related to MTD).
In conclusion, in this routine public health setting, MTD performance characteristics were similar to those previously reported under “study” conditions. Incorporation of MTD testing into routine public health laboratory algorithms could aid in the initial management of PTB suspects.
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Outcomes of the Amplified Mycobacterium tuberculosis Direct Test in a Public Health Laboratory