Smear-positive Respiratory Specimens Research Articles

Clinical Impact of Nucleic Acid Amplification Testing in the Diagnosis of Mycobacterium Tuberculosis: A 10-Year Longitudinal Study.

BackgroundNucleic acid amplification (NAA) testing for Mycobacterium tuberculosis (MTB) offers improved diagnostic accuracy, compared with smear microscopy, in differentiating MTB from other mycobacteria. We aimed to evaluate the reliability and projected impact of NAA testing in patients with acid-fast bacilli (AFB) smear-positive respiratory samples.MethodsWe identified a retrospective cohort of all patients with AFB smear-positive respiratory specimens at Henry Ford Hospital from January 1, 2001 through December 31, 2011. We examined the association between patients’ sociodemographic factors and clinical comorbidities with the likelihood of being diagnosed with MTB. We evaluated the projected change in duration of airborne isolation and unnecessary MTB treatment with introducing NAA testing into clinical decision making for AFB smear-positive patients.ResultsOne hundred thirty patients had AFB smear-positive respiratory specimens, 80 of these patients had a positive NAA test result, and 82 patients grew MTB on culture. Nucleic acid amplification testing had a sensitivity and specificity of 97.6% and 100%, respectively. Integrating NAA testing into clinical decision making for patients with AFB-positive smears was associated with a significantly shorter time in airborne isolation (6.0 ± 7.6 vs 23.1 ± 38.0, P < .001) and 9.5 ± 11.32 fewer days of unnecessary MTB treatment in patients with negative NAA test.ConclusionsNucleic acid amplification testing provided a rapid and accurate test in the diagnosis of MTB while significantly reducing the duration of isolation and unnecessary medications in patients with negative NAA test.

Evaluation of the Cobas TaqMan MTB test for detection of Mycobacterium tuberculosis complex

Background: The Cobas TaqMan MTB assay is used for rapid detection of the Mycobacterium tuberculosis complex (MTC) in clinical samples. It is only validated for respiratory samples, but is often requested by physicians for non-respiratory specimens. The aim of this study was therefore to evaluate the performance of this assay in clinical praxis in a country with low prevalence of tuberculosis (TB). Methods: The results from 2388 respiratory and 1005 non-respiratory human specimens were analyzed by this real-time PCR technique. Using culture results as the gold standard, the sensitivity, specificity, positive predictive value (PPV), and negative predictive values (NPV) of the PCR results were calculated. Results: For smear-positive respiratory specimens all the four values investigated were 100%. The number of smear-positive non-respiratory specimens was only eight, and all of these eight specimens reacted positively. Sensitivity was 51% for smear-negative respiratory specimens, and 47% for smear-negative non-respiratory specimens. When all non-respiratory specimens were analyzed together the sensitivity was 51%. Specimens from 16 patients were PCR-positive but culture-negative and these cases are discussed. In one case, TB DNA was still present in sputum 2 years after a successful treatment. Conclusion: The study shows that the Cobas TaqMan MTB assay performs well in smear-positive samples, while the sensitivities are unsatisfactory for both respiratory and non-respiratory smear-negative specimens. Furthermore, the analyses emphasize that this assay cannot be used to evaluate treatment or contagiousness, or to detect relapses or screen for TB.

Cost analysis of a nucleic acid amplification test in the diagnosis of pulmonary tuberculosis at an urban hospital with a high prevalence of TB/HIV.

IntroductionThe Centers for Disease Control and Prevention has recommended using a nucleic acid amplification test (NAAT) for diagnosing pulmonary tuberculosis (TB) but there is a lack of data on NAAT cost-effectiveness.MethodsWe conducted a prospective cohort study that included all patients with an AFB smear-positive respiratory specimen at Grady Memorial Hospital in Atlanta, GA, USA between January 2002 and June 2008. We determined the sensitivity, specificity, and positive and negative predictive value of a commercially available and FDA-approved NAAT (amplified MTD, Gen-Probe) compared to the gold standard of culture. A cost analysis was performed and included costs related to laboratory tests, hospital charges, anti-TB medications, and contact investigations. Average cost per patient was calculated under two conditions: (1) using a NAAT on all AFB smear-postive respiratory specimens and (2) not using a NAAT. One-way sensitivity analyses were conducted to determine sensitivity of cost difference to reasonable ranges of model inputs.ResultsDuring a 6 1/2 year study period, there were 1,009 patients with an AFB smear-positive respiratory specimen at our public urban hospital. We found the NAAT to be highly sensitive (99.6%) and specific (99.1%) on AFB smear-positive specimens compared to culture. Overall, the positive predictive value (PPV) of an AFB smear-positive respiratory specimen for culture-confirmed TB was 27%. The PPV of an AFB smear-positive respiratory specimen for culture-confirmed TB was significantly higher for HIV-uninfected persons compared to those who were HIV-seropositive (152/271 [56%] vs. 85/445 [19%]; RR = 2.94, 95% CI 2.36–3.65, p<0.001). The cost savings of using the NAAT was $2,003 per AFB smear-positive case.ConclusionsRoutine use of the NAAT on AFB smear-positive respiratory specimens was highly cost-saving in our setting at a U.S. urban public hospital with a high prevalence of TB and HIV because of the low PPV of an AFB smear for culture-confirmed TB.

Performance of Cobas TaqMan MTB for Detection of Mycobacterium tuberculosis Complex in Respiratory Specimens

Objective: Because tuberculosis is a major global health problem, rapid and accurate identification of Mycobacterium tuberculosis complex (MTBC) in clinical specimens is essential for the control of the spread of tuberculosis. One of real-time polymerase chain reaction assays for direct detection of MTBC, Cobas TaqMan MTB (Roche Diagnostics) assay has been widely used in clinical setting in Korea. The aim of study was to evaluate of the performance of Cobas TaqMan MTB assay in respiratory specimens. Methods: Total of 619 respiratory specimens (572 sputum specimens and 47 bronchoalveolar washing fluid specimens) was collected in August and September, 2011. All specimens were analyzed for the detection of MTBC by direct smear examination, mycobacterial culture, and Cobas TaqMan MTB assay. Results: When the culture method was used as the gold standard test for comparison, Cobas TaqMan MTB assay for detection of MTBC had a sensitivity and specificity of 80.9% (38/47) and 100% (572/572), respectively. Positive predictive value and negative predictive value of Cobas TaqMan MTB assay was 100% and 98.5%, respectively. The sensitivity of Cobas TaqMan MTB assay was 100% in AFB smear-positive samples and 59.1% in smear-negative samples. Conclusion: Cobas TaqMan MTB assay has an excellent performance in smear-positive respiratory specimens. But performance of Cobas TaqMan MTB assay in smear-negative respiratory samples was not good. For detection of paucibacillary tuberculosis, conventional mycobacterial culture must not be replaced by Cobas TaqMan MTB assay.

Evaluation of the Speed-oligo Direct Mycobacterium tuberculosis Assay for Molecular Detection of Mycobacteria in Clinical Respiratory Specimens

We present the first evaluation of a novel molecular assay, the Speed-oligo Direct Mycobacterium tuberculosis (SO-DMT) assay, which is based on PCR combined with a dipstick for the detection of mycobacteria and the specific identification of M. tuberculosis complex (MTC) in respiratory specimens. A blind evaluation was carried out in two stages: first, under experimental conditions on convenience samples comprising 20 negative specimens, 44 smear- and culture-positive respiratory specimens, and 11 sputa inoculated with various mycobacterium-related organisms; and second, in the routine workflow of 566 fresh respiratory specimens (4.9% acid-fast bacillus [AFB] smear positives, 7.6% MTC positives, and 1.8% nontuberculous mycobacteria [NTM] culture positives) from two Mycobacterium laboratories. SO-DMT assay showed no reactivity in any of the mycobacterium-free specimens or in those with mycobacterium-related organisms. Compared to culture, the sensitivity in the selected smear-positive specimens was 0.91 (0.92 for MTC and 0.90 for NTM), and there was no molecular detection of NTM in a tuberculosis case or vice versa. With respect to culture and clinical data, the sensitivity, specificity, and positive and negative predictive values for the SO-DMT system in routine specimens were 0.76 (0.93 in smear positives [1.0 for MTC and 0.5 for NTM] and 0.56 in smear negatives [0.68 for MTC and 0.16 for NTM]), 0.99, 0.85 (1.00 in smear positives and 0.68 in smear negatives), and 0.97, respectively. Molecular misidentification of NTM cases occurred when testing 2 gastric aspirates from two children with clinically but not microbiologically confirmed lung tuberculosis. The SO-DMT assay appears to be a fast and easy alternative for detecting mycobacteria and differentiating MTC from NTM in smear-positive respiratory specimens.

Clinical Evaluation of TRCRapid M.TB for Detection of Mycobacterium tuberculosis Complex in Respiratory and Nonrespiratory Specimens

The rapid and accurate diagnosis of tuberculosis is crucial to providing optimal treatment and reducing the spread of infection. We evaluated respiratory and nonrespiratory clinical specimens using a new automated Mycobacterium tuberculosis complex (MTBC) rRNA detection kit (TRCRapid M.TB; Tosoh Bioscience, Tokyo, Japan), which is based on the transcription-reverse transcription concerted reaction (TRC). TRC enables the rapid and completely homogeneous real-time monitoring of isothermal RNA sequence amplification without any postamplification procedures. The results were compared with those obtained by M. tuberculosis culture. A total of 1,155 respiratory specimens and 420 nonrespiratory specimens collected from 1,282 patients were investigated. Of the 45 specimens culture positive for MTBC, 42 were TRC positive, and of the 1,530 specimens culture negative for MTBC, 1,523 were TRC negative. Compared to the results of culture, the overall sensitivity and specificity of TRC were 96.6% and 99.9%, respectively, for respiratory specimens and 87.5% and 98.5%, respectively, for nonrespiratory specimens. The sensitivities of TRC were 100% for smear-positive respiratory and nonrespiratory specimens, 88.9% for smear-negative respiratory specimens, and 80% for smear-negative nonrespiratory specimens. No significant differences in test performance between respiratory and nonrespiratory specimens were observed. The TRC method proved to be clinically useful for the rapid identification of MTBC in respiratory and nonrespiratory specimens and in both smear-positive and smear-negative samples.

Clinical significance of isolation of nontuberculous mycobacteria in pulmonary tuberculosis patients

Clinical significance of isolation of nontuberculous mycobacteria (NTM) from respiratory specimens in patients with pulmonary tuberculosis is unknown. This study aimed to investigate the prevalence and clinical impact of NTM in pulmonary tuberculosis patients. We retrospectively reviewed data of patients with culture-confirmed pulmonary tuberculosis from 2000 to 2006. Those in whom NTM were isolated from respiratory specimens collected within two months before and nine months after the index culture of tuberculosis was plated were compared with those without NTM. Patients were followed for one year after initiation of anti-tuberculous treatment. Among 2133 patients with pulmonary tuberculosis, 48 (2.3%) with multiple and 106 (5.0%) with one isolate(s) of NTM were identified. Another 144 without NTM were selected and compared. The one-year mortality rates were similar among three groups. Patients with multiple NTM isolates were more likely to be symptomatic, sought medical help earlier, had smear-positive respiratory specimens, and received anti-tuberculous treatment later than those with single/no isolate(s). Cavities were more commonly visualized radiographically in patients with multiple/single NTM isolate(s) than those without NTM isolates. The mean duration of anti-tuberculous treatment was 8.8 months for patients with multiple NTM isolates, significantly longer than that in patients with single (7.6 months) and no NTM isolate(s) (7.5 months). NTM were present in the respiratory tract of 7.3% of patients with pulmonary tuberculosis. Although the outcomes were similar, the presence of NTM was associated with different clinical manifestations and had a significant impact on the treatment of tuberculosis.

Evaluation of the GenoType Mycobacteria Direct assay for the simultaneous detection of the Mycobacterium tuberculosis complex and four atypical mycobacterial species in smear-positive respiratory specimens

A novel, commercially available reverse hybridization assay [GenoType Mycobacteria Direct (GTMD), version 2.0; Hain Lifescience] was evaluated for the direct detection of five clinically relevant mycobacterial species [Mycobacterium tuberculosis complex (MTBC), Mycobacterium avium, Mycobacterium malmoense, Mycobacterium kansasii and Mycobacterium intracellulare] from 54 smear-positive respiratory specimens and the findings were compared with culture results. Three approaches were used for specimen preparation using either whole or 'split' sample volumes and N-acetyl-l-cysteine/3 % NaOH or 4 % NaOH as decontamination chemicals. Forty-three out of 52 samples in which RNA amplification was successful gave GTMD results that concurred with the identification of the cultured isolate. All cases of MTBC were detected. Twenty-two samples contained M. tuberculosis complex, seven had M. kansasii, four had M. malmoense, seven contained atypical mycobacteria other than those detectable using the GTMD assay and three specimens contained no viable mycobacteria. The assay is easy to use and can be completed in one working day. Results interpretation is facilitated by the inclusion of an internal amplification control with each sample to allow identification of specimens containing amplification inhibitors. A positive GTMD result will quickly identify patients with MTBC infection or provide specific identification of four other atypical mycobacteria from the same specimen. This allows more rapid drug susceptibility testing, treatment, and public health and infection control decisions.

Characteristics distinguishing disseminated Mycobacterium tuberculosis (MTB) and non-tuberculous mycobacterial (NTM) infection in HIV patients

of results Of the 64 patients with confirmed mycobacterial infection, 31 had disseminated disease (MTB:21, NTM:10 [nine due to M. avium Complex]). Of these, 81% were black African, 45% male, and median age was 34 years. Patients with NTM had lower CD4 T-cell counts, more prior or concurrent AIDS-defining illnesses, and were more frequently taking HAART at the time of mycobacterial diagnosis. Six NTM infections and one MTB infection appeared to be unmasked by HAART. Clinical features at presentation were non-discriminatory. On imaging, patients with MTB more often had parenchymal lung disease, whilst thoracic and abdominal lymphadenopathy was common in both groups. None of the patients with NTM had AFB smear positive respiratory specimens. Whereas the yield of blood cultures was low in patients with MTB, bone marrow specimens were diagnostic in all patients. Mortality was 8% for MTB and 30% for NTM cases. Immune reconstitution disease was observed in 52% of MTB and 30% of NTM patients. (Table 1.)

Rapid Genotypic Detection of Rifampin- and Isoniazid-Resistant Mycobacterium tuberculosis Directly in Clinical Specimens

A multiplex PCR DNA strip assay (Genotype MTBDR) designed to detect rifampin (rpoB) and high-level isoniazid (katG) resistance mutations in Mycobacterium tuberculosis isolates was optimized for clinical specimens. Successful genotypic results were achieved with 36 of 38 (95%) smear-positive respiratory specimens, allowing rapid therapeutic adjustments in transmittable drug-resistant tuberculosis.

Direct Application of the INNO-LiPA Rif.TB Line-Probe Assay for Rapid Identification of Mycobacterium tuberculosis Complex Strains and Detection of Rifampin Resistance in 360 Smear-Positive Respiratory Specimens from an Area of High Incidence of Multidrug-Resistant Tuberculosis

The INNO-LiPA Rif.TB assay for the identification of Mycobacterium tuberculosis complex strains and the detection of rifampin (RIF) resistance has been evaluated with 360 smear-positive respiratory specimens from an area of high incidence of multidrug-resistant tuberculosis (MDR-TB). The sensitivity when compared to conventional identification/culture methods was 82.2%, and the specificity was 66.7%; the sensitivity and specificity were 100.0% and 96.9%, respectively, for the detection of RIF resistance. This assay has the potential to provide rapid information that is essential for the effective management of MDR-TB.

The other face of Janus.

Adalberto LaszloIn this issue, M.H.Hazbon (1) reviews recentadvances in molecular methods for early diagnosisof tuberculosis and drug-resistant tuberculosis.This article has much in common with other “stateof the art” reviews that have been appearing inthe world’s literature on molecular tuberculosis(TB) diagnostics for some time now. Most of thesereviews are well written, factual accounts ofadvances in this important field of research. Inthe last two decades much has been stated andeven more has been promised about the role ofgenomics in providing alternatives to theconventional methods that have been used forover a century.Emphasis is often given to the development ofnew diagnostic tools with turn-around timesmeasured in hours rather than days, weeks ormonths. However, in practical terms, very little ofwhat is often described with such vigorousenthusiasm offers actual relief to the modern TBlaboratory specialists or indeed much of achallenge to the old, conventional, smearmicroscopy and drug susceptibility testing (DST).As the author of the above article laments, onlytwo of these methods, based on nucleic acidamplification and detection of specificmycobacterial sequences and mutations, havebeen approved (2,3) by the Food and DrugAdministration (FDA) in the USA.Moreover, the approval for the test was given onlyfor AFB smear-positive respiratory specimens inthe case of the Amplicor test, and for either smearpositive or negative respiratory specimens in thecase of the Enhanced Mycobacteriumtuberculosis test (E-MTD). This is far from thestellar performance that the sophistication of thetechnologies used could have allowed us to expectand indeed demand. However, the recentsequencing of M. tuberculosis’s genome willeventually lead to the deciphering of the meaningof much genetic information and the creation oftruly novel TB diagnostics.The status of the replacements for the othermainstay of TB diagnostics, i.e. drug susceptibilitytesting, is even less advanced. Not only there hasnot been regulatory approval for the new tests butit has become evident over time “that the presenceor absence of mutations in drug resistanceassociated genes does not necessarily indicatesusceptibility or resistance to the correspondingdrug” (1). Isolates with mutations in DR-associatedgenes can show phenotypic susceptibility andviceversa. The so-called “real resistance-mutations” can be clinically relevant, but theirabsence doesn’t necessarily mean drugsusceptibility. These circumstances mean that testresults obtained with these methods must alwaysbe confirmed by results obtain phenotypicmethods (4-6).The existence of multiple mutations conferring re-sistance to a given drug have dampened earlyenthusiasm for these methods. Even DNAsequencing, the “gold standard” for the detectionof drug mutations, requires several sequencingreactions per isolate, and is unlikely to be usedroutinely, except for rifampicin. Otherdisadvantages include high costs, the need forsophisticated equipment and expertise, thetechnical difficulty in the performance of this testsand frequent contamination problems. Had someof these tests been developed half a century ago,physicians would have been clamouring for a more

Cost-effectiveness analysis of the gen-probe amplified mycobacterium tuberculosis direct test as used routinely on smear-positive respiratory specimens.

A decision analysis was conducted to evaluate the cost-effectiveness of programs in which the Amplified Mycobacterium Tuberculosis Direct test (MTD) (Gen-Probe) is used to rapidly exclude Mycobacterium tuberculosis complex as a cause of disease in smear-positive respiratory specimens. MTD sensitivity, specificity, and probability of inhibition for smear-positive specimens were estimated from literature reports. Costs and laboratory performance characteristics were determined from review of records and practices at an urban hospital in the mid-Atlantic United States. In the base case, 31.4% of smear-positive specimens were assumed to be culture positive for M. tuberculosis. Under these conditions, the marginal cost of the MTD testing program was estimated as $338 per smear-positive patient, or $494 per early exclusion of tuberculosis based on negative MTD results. By comparison, the cost of respiratory isolation ($27.77/day) and drugs ($5.66/day) averted by MTD testing was estimated at $201 per early tuberculosis exclusion. MTD testing was therefore not cost-effective in this scenario. Sensitivity analysis revealed that cost-effectiveness estimates are sensitive to the number of smear-positive specimens processed annually, the relative prevalence of M. tuberculosis in smear-positive specimens, and the marginal daily cost of respiratory isolation. A decision tool is therefore presented for assessing the cost-effectiveness of MTD under various combinations of those three variables. While routine MTD testing of smear-positive specimens is not expected to be cost-saving for most individual hospitals, centralized reference laboratories may be able to implement MTD in a cost-effective manner across a wide range of situations.

The Cobas Amplicor MTB test for detection of Mycobacterium tuberculosis complex from respiratory and non-respiratory clinical specimens.

The Cobas Amplicor MTB test is a polymerase chain reaction (PCR) technique commonly used for direct detection of Mycobacterium tuberculosis in clinical samples. This assay is only validated for respiratory specimens, but many physicians also request PCR analyses for non-respiratory ones. 877 respiratory and 564 non-respiratory specimens were analysed by this test. Using culture results as standard, the sensitivity, specificity, positive (PPV) and negative predictive values (NPV) of the PCR were, respectively, 97.9%, 100%, 100% and 94.4% for smear-positive respiratory specimens, 68.8%, 99.2%, 87.5% and 97.5% for smear-negative respiratory samples, 57.8%, 98.6%, 78.8% and 96.4% for all non-respiratory specimens, and 42.4%, 98.6%, 66.7% and 96.4% for smear-negative non-respiratory specimens. 154 cerebrospinal fluid samples were analysed and the sensitivity, specificity, PPV and NPV were 55.6%, 97.2%, 55.6% and 97.2%, respectively. These results indicate that the Cobas Amplicor MTB test enables detection of tuberculosis in respiratory specimens, but does not perform well enough in non-respiratory specimens. The method fails particularly in cases where a reliable and rapid test is urgently needed, e.g. in tuberculous meningitis.

Rapid and Specific Detection of Mycobacterium tuberculosis from Acid-Fast Bacillus Smear-Positive Respiratory Specimens and BacT/ALERT MP Culture Bottles by Using Fluorogenic Probes and Real-Time PCR

A real-time PCR assay using the LightCycler (LC) instrument for the specific identification of Mycobacterium tuberculosis complex (MTB) was employed to detect organisms in 135 acid-fast bacillus (AFB) smear-positive respiratory specimens and in 232 BacT/ALERT MP (MP) culture bottles of respiratory specimens. The LC PCR assay was directed at the amplification of the internal transcribed spacer region of the Mycobacterium genome with real-time detection using fluorescence resonance energy transfer probes specific for MTB. The results from the respiratory specimens were compared to those from the Amplicor M. tuberculosis PCR test. Specimens from MP culture bottles were analyzed by Accuprobe and conventional identification methods. MTB was cultured from 105 (77.7%) respiratory AFB smear-positive specimens; 103 of these samples were positive by LC PCR and Amplicor PCR. Two samples negative in the LC assay contained rare numbers of organisms; both were positive in the Amplicor assay. Two separate samples negative by Amplicor PCR contained low and moderate numbers of AFB, respectively, and both of these were positive in the LC assay. There were 30 AFB smear-positive respiratory specimens that grew mycobacteria other than tuberculosis (MOTT), and all tested negative in both assays. Of the 231 MP culture bottles, 114 cultures were positive for MTB and all were positive by the LC assay. The remaining 117 culture bottles were negative in the LC assay and grew various MOTT. This real-time MTB assay is sensitive and specific; a result was available within 1 h of having a DNA sample available for testing.

Evaluation of Gen-Probe amplified mycobacterium tuberculosis direct test by using respiratory and nonrespiratory specimens in a tertiary care center laboratory.

The performance of the Amplified Mycobacterium Tuberculosis Direct (AMTD) test (Gen-Probe Inc., San Diego, Calif.) was assessed in a large tertiary care mycobacteriology laboratory. Both acid-fast smear-positive and smear-negative respiratory and nonrespiratory clinical specimens were analyzed. From February 1998 to 4 October 2001, AMTD assays were performed on 391 respiratory specimens and 164 nonrespiratory specimens. The AMTD assay was compared to the "gold standard" of combined culture and clinical diagnosis. The overall sensitivity for all specimens, including those for which no smear result was available, was 91.2%. The overall sensitivities of the assay, including acid-fast smear-positive and -negative specimens, were 97.8 and 77.3% for respiratory and nonrespiratory specimens, respectively. The corresponding specificities for respiratory and nonrespiratory specimens were 99.1 and 98.5%, respectively. The overall specificity for all specimens was 98.9%. Positive and negative predictive values were 93.9 and 99.7% and 91.7 and 96.4% for respiratory and nonrespiratory specimens, respectively. The time saved by using the AMTD test for making a diagnosis of tuberculosis instead of using culture was 8.99 days. Inhibitors to the AMTD assay were found in 3.1% of respiratory specimens and 3.1% of nonrespiratory specimens. The assay, used in a general mycobacteriology laboratory setting, represents an important advance in improving the speed and accuracy of diagnosis in the management of patients with tuberculosis.

Relationship Between Estimated Pretest Probability and Accuracy of Automated Mycobacterium tuberculosis Assay in Smear-Negative Pulmonary Tuberculosis

The AMPLICOR assay (Roche; Branchburg, NJ), a rapid direct amplification test for Mycobacterium tuberculosis, has only been licensed for use in smear-positive respiratory specimens. However, many patients with pulmonary tuberculosis (PTB) have smear-negative disease. The clinical utility of this test in patients with smear-negative PTB is unknown. To evaluate the effect of pretest probability of PTB estimated by chest physicians on the accuracy of the AMPLICOR assay in patients with smear-negative PTB. A prospective study of consecutive patients suspected of having smear-negative PTB. Two chest physicians estimated the pretest probability of active disease (high, intermediate, and low categories). Respiratory specimens were examined with radiometric broth medium cultures and with the AMPLICOR assay for M tuberculosis. The decision on a final diagnosis of PTB was blinded to the AMPLICOR results. Active PTB was diagnosed in 25 of 441 patients (5.7%). The AMPLICOR assay had an overall sensitivity of 44% and a specificity of 99%. Results of the assay were negative in seven patients with culture-negative PTB. The proportions of patients in the high, intermediate, and low pretest groups were 4.5%, 19.7%, and 75.7%, respectively. The incidence of PTB for each group was 95%, 3.4%, and 0.9%, respectively. The sensitivities of the AMPLICOR assay in the three groups of patients were 47%, 33%, and 33%, respectively, while the specificities were 100%, 98%, and 99%, respectively. In patients suspected of having smear-negative PTB, the following conclusions were drawn: (1) the incidence of active PTB was low; (2) pretest estimates accurately discriminated between patients with high and low risk of PTB; (3) the risk of PTB was overestimated in the intermediate group; and (4) the utility of the AMPLICOR assay in the intermediate-risk group may be limited by the overestimation of disease prevalence and low test sensitivity. Further studies are needed on the role of the AMPLICOR assay in better selected patients with an intermediate risk of having smear-negative PTB.

Update: Nucleic Acid Amplification Tests for Tuberculosis

On September 30, 1999, the Food and Drug Administration approved a reformulated Amplified Mycobacterium Tuberculosis Direct Test (MTD) (Gen-Probe, San Diego, California) for detection of Mycobacterium tuberculosis in acid-fast bacilli (AFB) smear-positive and smear-negative respiratory specimens from patients suspected of having tuberculosis (TB). MTD and one other nucleic acid amplification (NAA) test, the Amplicor Mycobacterium Tuberculosis Test (Amplicor) (Roche Diagnostic Systems, Inc., Branchburg, New Jersey), previously had been approved for the direct detection of M. tuberculosis in respiratory specimens that have positive AFB smears. This notice updates the original summary published in 1996 (1) and provides suggestions for using and interpreting NAA test results for managing patients suspected of having TB. The appropriate number of specimens to test with NAA will vary depending on the clinical situation, the prevalence of TB, the prevalence of nontuberculous mycobacteria (NTM), and laboratory proficiency (2,3). Based on available information, the following algorithm is a reasonable approach to NAA testing of respiratory specimens from patients with signs or symptoms of active pulmonary TB for whom a presumed diagnosis has not been established.

Multicenter evaluation of the Abbott LCx Mycobacterium tuberculosis ligase chain reaction assay.

Four Australian hospital laboratories evaluated the performance of the Abbott LCx Mycobacterium tuberculosis assay with 2,347 specimens (2,083 respiratory and 264 nonrespiratory specimens) obtained from 1, 411 patients. A total of 152 specimens (6.5%) were culture positive for Mycobacterium tuberculosis complex (MTBC); of these, 79 (52%) were smear positive. After resolution of discrepant data, the overall sensitivity, specificity, and positive and negative predictive values for the LCx assay were 69.7, 99.9, 99.1, and 97.7% respectively. For smear-positive respiratory specimens that were culture positive for MTBC, the values were 98.5, 100, 100, and 98.4%, respectively, while the values for smear-negative respiratory specimens were 41.5, 99.9, 96.4, and 98%, respectively. Relative operating characteristic curves were constructed to demonstrate the relationship between sensitivity and specificity for a range of possible cutoff values in the LCx assay. These graphs suggested that the assay sensitivity for respiratory samples could be increased from 70.2 to 78.6%, while the specificity would be reduced from 99.9 to 99.4% by inclusion of a grey zone (i.e., LCx assay values of between 0.2 and 0.99). An algorithm is presented for the handling of specimens with LCx assay values within this grey zone.

Direct detection of Mycobacterium tuberculosis complex in respiratory specimens by a target-amplified test system

A total of 938 respiratory specimens (633 sputa, 249 bronchial and tracheal aspirates, and 56 bronchoalveolar lavages) from 589 patients were tested for direct detection of Mycobacterium tuberculosis complex by the Gen-Probe amplified Mycobacterium tuberculosis direct test (MTD), and the results were compared with those of the conventional methods of fluorescence microscopy and cultivation (solid and radiometric media). One series of specimens (n = 515) was decontaminated with N-acetyl-L-cysteine (NALC)-NaOH: the other one (n = 423) was decontaminated with sodium dodecyl (lauryl) sulfate (SDS)-NaOH. Of the specimens decontaminated with NALC, 39 were MTD and culture positive, 455 were MTD and culture negative, 18 were MTD positive and culture negative, and 3 were MTD negative and culture positive, indicating a sensitivity of 92.9% and a specificity of 96.2% for the MTD. Of the specimens decontaminated with SDS, 35 were MTD and culture positive, 372 were MTD and culture negative, 15 were MTD positive and culture negative, and 1 was MTD negative and culture positive, indicating a sensitivity of 97.2% and a specificity of 96.1% for the MTD. After resolution of discrepant results by review of the patients' clinical data, the sensitivity of the MTD was 93.9%, the specificity was 97.6%, the positive predictive value was 80.7%, and the negative predictive value was 99.3% for the NALC series; the corresponding values were 97.4, 96.9, 76.0, and 99.7%, respectively, for the SDS series. In conclusion, the MTD is a highly sensitive and specific technique for detecting M. tuberculosis complex within hours in both smear-positive and smear-negative respiratory specimens.