Cobas Amplicor Assay Research Articles

Evaluation of Quantification of HIV-1 RNA Viral Load in Plasma and Dried Blood Spots by Use of the Semiautomated Cobas Amplicor Assay and the Fully Automated Cobas Ampliprep/TaqMan Assay, Version 2.0, in Kisumu, Kenya

In Kenya, HIV-1 viral load monitoring is commonly performed with the Cobas Amplicor using plasma specimens. Interest is growing in transitioning to real-time PCR (RT-PCR), such as the Cobas Ampliprep/Cobas TaqMan (CAP/CTM), using dried blood spots (DBS). Before implementation, direct evaluation of the two assays using DBS field specimens is required. This study compares the sensitivity, specificity, negative and positive predictive values (NPV and PPV, respectively), concordance, and agreement between HIV-1 viral load measurements using plasma and DBS specimens obtained from 512 HIV-1-infected pregnant females enrolled in the Kisumu Breastfeeding Study and tested with the Cobas Amplicor and CAP/CTM assays. The sensitivity and NPV of viral load detection in DBS specimens were higher with CAP/CTM (sensitivity, 100%; 95% confidence interval [CI], 99.1 to 100.0%; NPV, 100%; 95% CI, 59.0 to 100.0%) than the Cobas Amplicor (sensitivity, 96.6%; 95% CI, 94.3 to 98.1%; NPV, 58.8%; 95% CI, 40.7 to 75.4%). The PPVs were comparable between both assays when using DBS. The specificity of viral load detection in DBS specimens was lower with CAP/CTM (77.8%; 95% CI, 40.0 to 97.2%) than that of the Cobas Amplicor (95.2%; 95% CI, 76.2 to 99.9%). Good concordance and agreement were observed when paired plasma and DBS specimens were tested with both assays. Lower specificity with the CAP/CTM is likely due to proviral HIV-1 DNA amplification and lower detection limits with RT-PCR. However, the CAP/CTM has better sensitivity and higher throughput than the Cobas Amplicor. These findings suggest that DBS may be a suitable alternative to plasma when using RT-PCR, which could increase access to viral load monitoring in resource-limited settings.

Rapid Detection of the Mycobacterium tuberculosis Complex by Use of Quenching Probe PCR (geneCube)

Early detection of tuberculosis (TB) is essential for infection control. The geneCube (Toyobo) is a novel fully automated gene analyzer that can amplify target DNAs within 60 min. In this study, we evaluated the ability of the geneCube to directly detect Mycobacterium tuberculosis complex (MTBC) and Mycobacterium avium complex (MAC) in clinical specimens. The results were then compared with those obtained using conventional culture, microscopy, and the Cobas Amplicor assay (Roche). We examined a total of 516 frozen samples from 69 patients who showed culture-positive infection (73 samples; 39 MTBC, 32 MAC, and 2 mixed infections) and from 354 patients who were culture negative (443 samples). Assays using the geneCube had a sensitivity of 85.4% and a specificity of 99.8% for detection of MTBC and a sensitivity of 85.3% and a specificity of 99.8% for detection of MAC. These results are similar to those obtained using the Amplicor system but were obtained much more rapidly (1 h with the geneCube versus 5.5 h with the Amplicor system). The geneCube thus enables a significant shortening of the assay time with no loss of sensitivity or specificity.

Detection of tuberculosis using artus® M. tuberculosis PCR Kit and COBAS® AMPLICOR™ Mycobacterium tuberculosis Test

Nucleic acid amplification tests can detect Mycobacterium tuberculosis complex rapidly and reliably. To compare the diagnostic performance of the artus M. tuberculosis PCR Kit and COBAS AMPLICOR Mycobacterium tuberculosis Test. In the artus assay, an appropriate cycle threshold (Ct) value was determined for positivity. A total of 238 clinical respiratory specimens were analysed using both the artus and COBAS AMPLICOR assays. In 221 specimens, these results were further compared with culture results. The overall agreement between artus and COBAS AMPLICOR was 96.2% (229/238). Among the nine (3.8%) discrepant specimens, three (1.3%) were artus-positive and COBAS AMPLICOR-negative, while the other six (2.5%) were artus-negative and COBAS AMPLICOR-positive. Using culture as a standard, the sensitivity and specificity of the artus assay were 97.8% and 85.1%, and those of COBAS AMPLICOR assay were 100% and 86.2%, respectively. The difference was not statistically significant. In the artus assay, the minimum Ct value for the positivity determination was 38. The artus and COBAS AMPLICOR assays showed comparable diagnostic performance and can be confidently used for detection of M. tuberculosis complex. In the artus assay, a Ct value of 38 could be suggested as an appropriate cut-off value.

Monitoring of cytomegalovirus infection after allogeneic stem cell transplantation

More than 90% of worldwide population is infected with human cytomegalovirus (CMV), one of the most common agents which complicate immunocompromised patients. Viral infections, in particular CMV ones are still a major cause of moratality and morbidity after stem cell transplantation (SCT). Monitoring is performed by detecting CMV-Ag or virus DNA in peripheral blood. Risk factors are donor/recipient CMV status, type of transplant and acute graft versus host disease. The aim of the study was to determine the extent of validity of CMV infection monitoring after transplantation as a reliable parameter of further CMV replication course in patients with hematopoietic stem cell transplantation. A total of 49 patients with stem cell transplantation were studied prospectively during a 2-year period after transplantation for the presence of CMV DNA. Polymerase chain reaction (PCR) CMV DNA was performed on 222 full blood samples using Cobas Amplicor assay. Activation of CMV was detected in 10/49 (20.48%) of the patients. The median posttransplantation time for the first positive PCR result was 6 weeks for the stem cell transplant patients. Viremia became negative in all the cases after the antiviral therapy with ganciclovir. Our data show that the level of CMV-DNA load at the time of initial CMV detection after transplantation could be a possible predictor for further course of CMV replication in patients receiving hematopoietic stem cell.

Evaluation of the Cavidi ExaVir Load Assay (Version 3) for Plasma Human Immunodeficiency Virus Type 1 Load Monitoring

We evaluated the new low-cost ExaVir Load (version 3) reverse transcriptase viral load assay against the Roche Cobas Amplicor assay. Results for samples tested using the reverse transcriptase assay correlated well with those obtained with the Roche assay (r = 0.85; n = 202). The version 3 reverse transcriptase assay shows improved sensitivity compared to the previous version.

Evaluation of a novel internally controlled real-time PCR assay targeting the 16S rRNA gene for confirmation of Neisseria gonorrhoeae infections

A novel HybProbe real-time LightCycler PCR assay was developed for confirmation of Neisseria gonorrhoeae in samples positive according to the COBAS AMPLICOR Chlamydia trachomatis/Neisseria gonorrhoeae PCR assay. The new assay amplifies 375 bp of the N. gonorrhoeae 16S rRNA gene and includes an internal amplification control introduced during DNA purification. The assay had 100% specificity because of the high specificity of the HybProbes and primers. Other Neisseria spp. failed to generate positive crossing-point values and melting peaks. The analytical sensitivity for N. gonorrhoeae DNA was 0.5 fg/PCR, corresponding to 0.3 CFU/PCR. Sensitivity was not impaired in the presence of higher DNA concentrations (>or=1000-fold) from Neisseria spp. other than N. gonorrhoeae. The sensitivity was similar to that reported for the COBAS AMPLICOR assay with cervical swab samples. To assess its clinical applicability as a confirmatory test, 38 (2.9%) of 1313 swabs that were positive according to the COBAS AMPLICOR assay were tested using the new in-house assay and the commercially available GenFlow Neisseria test. Twenty-one samples negative according to COBAS AMPLICOR also underwent confirmatory testing. Both confirmatory tests yielded identical results; the 21 negative samples remained negative, and only 11 (28.9%) of the samples positive according to COBAS AMPLICOR were positive after retesting, suggesting a low prevalence (0.84%) of N. gonorrhoeae infection in the study population. These data suggest that the novel real-time PCR assay is an excellent and easy to interpret confirmatory test for the existing COBAS AMPLICOR assay for N. gonorrhoeae.

A comparison of three real-time PCR assays for the confirmation of Neisseria gonorrhoeae following detection of N. gonorrhoeae using Roche COBAS AMPLICOR

Three different real-time PCR assays were evaluated as confirmatory tests for Neisseria gonorrhoeae after initial screening using the COBAS AMPLICOR Chlamydia trachomatis and N. gonorrhoeae duplex assay. The target genes used for the confirmation were the gyr, cppB and 16S rRNA genes. Analytical specificity was determined by testing 60 strains belonging to different bacterial species and/or serogroups. The primers chosen from the 16S rRNA gene for confirmation of N. gonorrhoeae were highly specific, showed no cross-reactivity with other bacteria included in the study, and had an analytical sensitivity of 1 CFU. Of 192 clinical specimens that were positive for N. gonorrhoeae according to the COBAS AMPLICOR assay, 42 were confirmed as positive using the 16S rRNA gene target, 26 were confirmed using the cppB target, and 30 were confirmed using the gyr target. It was concluded that the real-time PCR assay targeting the 16S rRNA gene is a useful confirmatory assay to complement the COBAS AMPLICOR screening test for N. gonorrhoeae.

Diagnostic Value of Molecular Confirmation Assays for Neisseria gonorrhoeae

The COBAS Amplicor system (Roche Diagnostics, Rotkreuz, Switzerland) is a widely used screening test for infections with Neisseria gonorrhoeae due to its excellent sensitivity and ease of use. However, it has been found to cross-react with certain commensal Neisseria species (1, 3, 4). Therefore, an independent confirmation assay is mandatory. In the May 2007 issue of the Journal of Clinical Microbiology, Mangold et al. (2) presented a new confirmation assay which is capable of distinguishing between N. gonorrhoeae and Neisseria spp. in comparison to confirmation assays targeting the porA pseudogene and the cppB gene. They obtained positive results in their in-house assay for 102/181 (56%) COBAS Amplicor-positive samples; the porA confirmation assay was positive for 69/181 (38%) samples, and the cppB assay detected 115/181 (64%) positive samples. Additionally, Mangold et al. reported 15%, 2%, and 4% positive results for COBAS Amplicor-negative samples with the cppB, porA, and in-house assays, respectively. The presented data are discrepant from observations at our institution. Upon optimization of published detection protocols, we have compared the performances of three assays targeting the porA pseudogene (5), the cppB gene (6), and the 16S rRNA gene (in-house) for quality assurance. In samples positive for all three targets, the cppB confirmation assay was the most sensitive (mean crossing point [Cp], 25.9), followed by the porA assay (mean Cp, 26.2) and the 16S rRNA gene assay (mean Cp, 32.8). Over a period of 6 months, 398 samples yielded a positive COBAS Amplicor result (optical density [OD] of >0.2, as recommended by the manufacturer). Of these 398 samples, 258 samples (64.8%) were negative in all three confirmation assays. The remaining 140 samples (35.2%) yielded a positive result in at least two of the confirmation assays, with the majority being positive in all three assays (128/140 [91.4%]). Eight of 140 samples (5.7%) were negative in the 16S rRNA gene assay, probably due to small DNA amounts in the sample, as this assay is less sensitive than the other two confirmation assays, which is corroborated by the fact that 2/8 samples showed an OD value lower than 2.0 when initially tested in the COBAS Amplicor system. Four specimens were positive in the 16S rRNA gene and porA detection assays only and were interpreted as gonococci lacking the cppB-harboring plasmid (2.9%); the COBAS Amplicor value was above 2.0 OD in all four samples. Taken together for 136/140 (97.1%) specimens, the porA and cppB confirmation assays were concordantly positive. In contrast to the findings of Mangold et al., we did not obtain positive results for the confirmation assays when applied to COBAS Amplicor-negative samples. The conflicting results might in part be explained by an epidemiological distinct pool of N. gonorrhoeae isolates examined and an unfortunate study design, as modified interpretation guidelines for the COBAS Amplicor assay were used by Mangold et al. Samples were considered positive if two out of three runs gave a positive signal above an OD of 2.0, as opposed to one sample above an OD of 0.2 being sufficient. This decreases the number of positive samples and inevitably has a negative impact on the sensitivity of the screening assay. Presumably, this protocol accounts for the “false”-positive results of the cppB plasmid assay (33/227). It would be interesting if these samples were initially positive in the COBAS Amplicor system (OD of >0.2) by use of the manufacturer's interpretation criteria. Our analysis further cannot confirm the low sensitivity of the porA detection assay, as the majority of our specimens yielded positive results in all confirmation assays. Regarding the in-house NsppID assay published by Mangold et al., the design of the test itself is not completely conclusive: N. gonorrhoeae displays 100% homology with strains of Neisseria polysaccharea at the binding site of the detection probe (referred to as Npolysaccharea1 in the publication). N. polysaccharea was not tested for cross-reactivity in the NsppID assay. By use of an NCBI BLAST search, strains of Neisseria meningitidis also display 100% homology in this region of the 16S rRNA gene. Additionally, coamplification in PCRs should be avoided if no important information is obtained by the coamplified product, as it is likely to decrease sensitivity. A confirmation assay for N. gonorrhoeae that coamplifies apathogenic Neisseria spp. does not provide any additional information of clinical relevance. The studies indicate that the use of molecular diagnostic assays for the detection of N. gonorrhoeae infection remains challenging and that the characterization of the prevalence of target genes in distinct epidemiological pools is important to ensure accurate diagnosis.

Clinical performance of an in-house real-time RT-PCR assay using a fluorogenic LUX™ primer for quantitation of human immunodeficiency virus type-1 (HIV-1)

The South African National Antiretroviral Treatment Guideline recommends the use of HIV viral load assays for routine monitoring of HIV-1 positive patients on Highly Active Antiretroviral Therapy (HAART). Approved commercial HIV-1 viral load assays are expensive for developing countries where a large number of patients are treated in the public sector. The evaluation of an in-house HIV-1 viral load assay (LUX assay) is described using 458 plasma specimens. Good specificity of the LUX assay was demonstrated using 50 seronegative plasma specimens. A group of 142 HIV-1 positive patients was used to assess the agreement between the LUX assay and the COBAS Amplicor assay. An intra class correlation (ICC) coefficient of 0.85 (CI 95%) indicated good agreement between the assays. The Bland–Altman model showed good agreement between the assays for ∼87% of the results (mean 0.03 [−1.26; 1.32], CI 95%). In a cohort of 55 patients followed-up longitudinally the LUX assay showed similar declines in viral load to the COBAS Amplicor assay in response to therapy. Viral rebound was detected in 5 patients out of 55 by both assays. Thus, the LUX assay compares well to the gold standard and represents an affordable alternative for high volume testing in resource limited settings.

Neisseria Species Identification Assay for the Confirmation of Neisseria gonorrhoeae -Positive Results of the COBAS Amplicor PCR

Screening assays for Neisseria gonorrhoeae exhibit low positive predictive values, particularly in low-prevalence populations. A new real-time PCR assay that detects and identifies individual Neisseria spp. using melt curve analysis was compared to two previously published supplementary assays. NsppID, a 16S rRNA real-time PCR/melt curve assay developed to distinguish N. gonorrhoeae from other Neisseria spp., was compared to real-time PCR assays targeting genes reportedly specific for N. gonorrhoeae, the cppB gene and the porA pseudogene. A total of 408 clinical specimens (324 female endocervical swabs and 84 male urine or urogenital swab specimens) were screened using the COBAS Amplicor assay for Chlamydia trachomatis and N. gonorrhoeae (CT/NG) (Roche Diagnostics, Indianapolis, IN) followed by confirmatory testing via real-time PCR. The NsppID assay detected Neisseria spp. in 150/181 COBAS-positive specimens (82%), including six dual infections, and identified N. gonorrhoeae in 102 (56%) specimens. Sixty-nine of 181 (38%) specimens were positive for N. gonorrhoeae by porA pseudogene, and 115/181 (64%) were positive for cppB. However, cppB was also positive in 15% of COBAS-negative specimens, more than either NsppID (4%) or porA pseudogene (2%) assays. The porA pseudogene assay had the highest specificity for both genders but the lowest sensitivity, especially in female specimens. NsppID had a slightly lower specificity but greater sensitivity and overall accuracy. The least desirable confirmatory assay was cppB, due to poor specificity. The NsppID assay is an accurate confirmatory assay for N. gonorrhoeae detection. In addition, the NsppID assay can identify the non-N. gonorrhoeae species responsible for the majority of false-positive results from the COBAS Amplicor CT/NG assay.

Comparative performance of the Roche COBAS Amplicor assay and an in-house real-time PCR assay for diagnosis of Chlamydia trachomatis infection

This study investigated the comparative performance of the Amplicor assay and an in-house semi-automated, multiplex real-time PCR for the diagnosis of genital chlamydial infection. Four different assays, the COBAS Amplicor CT test (Amplicor PCR), in-house real-time PCR (IHRT-PCR), in-house nested cryptic plasmid PCR and in-house nested major outer membrane protein PCR, were performed on genital swabs from 1000 consecutive patients attending a genitourinary medicine clinic. The samples were designated true positive if Chlamydia trachomatis DNA was detected by at least two of the four above-mentioned assays while a sample was defined as true negative if C. trachomatis DNA was detected in only one or none of the assays. By this criterion, there were 129 true positive and 871 true negative samples for C. trachomatis DNA in this cohort. Amplicor PCR designated 144 samples positive: 128 (89%) of 144 samples were true positive and 16 (11%) were false positive. IHRT-PCR detected 126 of 129 true positive samples and did not generate any false positive results. The sensitivity of IHRT-PCR was comparable with, and specificity was higher than, Amplicor PCR for the diagnosis of genital chlamydial infection.

Comparison of Qualitative (COBAS AMPLICOR HCV 2.0 versus VERSANT HCV RNA) and Quantitative (COBAS AMPLICOR HCV Monitor 2.0 versus VERSANT HCV RNA 3.0) Assays for Hepatitis C Virus (HCV) RNA Detection and Quantification: Impact on Diagnosis and Treatment of HCV Infections

Quantitative measurements of serum hepatitis C virus (HCV) RNA are becoming increasingly important in the management of HCV-infected patients. Here we compared two quantitative assays, the COBAS AMPLICOR HCV Monitor 2.0 assay (Roche Diagnostics) and the branched DNA-based VERSANT HCV RNA 3.0 assay (Bayer Diagnostics) for HCV RNA measurement in 344 samples derived from 120 patients with chronic genotype 1 HCV infection. The overall concordance between the results of the two tests was 95%, and the HCV RNA titers within the dynamic ranges of the assays correlated very well (r2=0.86). Furthermore, both tests performed equally well in determining an early viral response at week 1 or 4 during antiviral therapy. We also compared two qualitative HCV RNA detection assays: the COBAS AMPLICOR HCV 2.0 assay versus the transcription-mediated amplification (TMA)-based VERSANT HCV RNA qualitative assay. Stored samples from sustained responders to interferon-ribavirin therapy were retested by the HCV TMA assay and were found to contain no detectable HCV RNA, demonstrating complete concordance between the results of PCR and TMA. However, HCV RNA was detected by the TMA assay in end-of-treatment (ETR) samples from 33% of patients with relapses who were HCV RNA negative according to the COBAS AMPLICOR assay. This observation suggests that a TMA assay can lead to a more correct definition of the ETR response.

Comparison of an Internally Controlled, Large-Volume LightCycler Assay for Detection of Mycobacterium tuberculosis in Clinical Samples with the COBAS AMPLICOR Assay

We present a sensitive and specific assay for reliable and flexible detection of members of the Mycobacterium tuberculosis complex (MTBC) in clinical samples. This real-time PCR assay, which uses the LightCycler 2.0 instrument and 100-mul glass capillaries, can provide a result within 1 h after DNA extraction. The primers amplify a 206-bp fragment of the MTBC 16S rRNA gene. The sensor hybridization probe targets a region highly specific to members of the MTBC. The assay also includes a novel type of internal control that monitors the function of the reaction components and can detect potential inhibitors. Template DNA was extracted by the same procedure used for the COBAS AMPLICOR M. tuberculosis assay, so the LightCycler assay could be directly compared to the COBAS AMPLICOR assay. The LightCycler assay was evaluated with 146 clinical samples of various types. Very good agreement (100% sensitivity, 98.6% specificity) could be shown between the LightCycler and COBAS AMPLICOR assays. Specificity was checked with a panel of nontuberculous mycobacteria, as well as a large panel of bacterial and fungal organisms.

Validation of roche COBAS Amplicor assay for detection of Chlamydia trachomatis in rectal and pharyngeal specimens by an omp1 PCR assay.

Screening guidelines for men who have sex with men (MSM) recommend testing of extragenital sites (pharyngeal and rectal) for gonorrhoea and chlamydia. Testing of specimens from these sites is not validated by most commercial nucleic amplification tests, such as the COBAS Amplicor assay. To investigate the utility of the COBAS Amplicor assay for detection of Chlamydia trachomatis in extragenital specimens, this study developed and evaluated confirmatory tests using the omp1 gene as an alternative target for amplification by PCR. Of anal and throat swabs collected from men in male-only saunas, 52 swabs that tested C. trachomatis positive by COBAS Amplicor and 30 swabs that tested as negative were included for confirmatory omp1 PCR testing. A total of 49 (94%) COBAS Amplicor-positive samples were confirmed by the omp1 PCR. A substantial proportion of specimens were confirmed by using a nested omp1 PCR (27%). Not confirmed by any omp1 PCR were three anal swabs (6%). It is most probable that these samples contained lower bacterial levels that were near or below the detection level of the omp1 PCR assays. The findings of this study support the confident reporting of C. trachomatis detected by COBAS Amplicor in extragenital specimens and support the utility of this assay as a screening test for MSM.

Role of Clinical Judgment in the Application of a Nucleic Acid Amplification Test for the Rapid Diagnosis of Pulmonary Tuberculosis

Several nucleic acid amplification (NAA) tests for Mycobacterium tuberculosis (MTB) have been licensed for the rapid diagnosis of active pulmonary tuberculosis (PTB) in respiratory secretions. There is uncertainty however regarding the practical application of these tests in clinical decision making. To evaluate the utility of the COBAS AMPLICOR assay (Roche Diagnostics; Singapore) for MTB as applied by specialists for the rapid diagnosis of PTB in the routine clinical setting. A prospective study of consecutive patients suspected of PTB and tested with the AMPLICOR assay under the care of respiratory physicians. The final diagnosis was based on all relevant clinical information after at least 3 months of follow-up. Accuracy of the NAA test was compared with that of the initial expectant treatment. Expectant treatment was based on an integrated approach that incorporated clinical evaluation with results of direct smear and NAA tests. The incidence of PTB in 168 patients was 32%. The basis for expectant treatment of PTB was positive smear result in 47%, clinical suspicion in 26%, and positive AMPLICOR result in 23%. The sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of the AMPLICOR test were 77%, 100%, 99%, 90%, and 93%, respectively. In comparison, they were 96%, 97%, 94%, 98%, and 97%, respectively, for the integrated clinical approach. In the rapid diagnosis of PTB, the clinical judgment of specialists augmented the utility of the NAA test: (1) specialists selected patients with high-to-moderate pretest probabilities, (2) they commenced treatment promptly on a positive NAA test result, and (3) they were willing to start treatment in some patients on the basis of high clinical suspicion despite negative smear and negative NAA test results.

The rapid diagnosis of smear‐negative pulmonary tuberculosis: A cost‐effectiveness analysis

The prompt diagnosis of smear-negative pulmonary tuberculosis (PTB) is a clinical challenge. It may be achieved by a number of tests which have varying accuracies, costs and degrees of invasiveness. The objective of this study was to compare the cost-effectiveness of clinical judgement (empirical), the Roche Cobas amplicor assay for Mycobacterium tuberculosis (amplicor), acid-fast staining of bronchoalveolar lavage specimens (BAL), nucleic acid amplification tests of bronchoalveloar lavage specimens for M. tuberculosis (BAL + NAA), computed tomography (CT) and amplicor assay followed by BAL. The range of predictive values of the various strategies were derived from published data and a new study of 441 consecutive adult patients with suspected smear-negative PTB prospectively stratified into three pretest risk groups: low, intermediate and high. The cost-effectiveness was evaluated with a decision tree model (DATA software). The incidence of PTB was 5.7% (4% culture positive) for the whole group, 95% in the high-risk group, 0.9% in the low-risk group and 3.4% in the intermediate-risk group. The sensitivity of the empirical approach was 49% and of the amplicor assay was 44%. Patient outcomes were expressed as life expectancy for the base case of a 58-year-old man with a pretest probability of 5.7%. At this low pretest risk the differences in life expectancies between tests was < 0.1 years and the empirical approach incurred the lowest cost. Sensitivity analysis at increasing pretest risks showed better life expectancies (approximately 1 years) for CT scan and test combinations than empirical and amplicor for additional costs of US$243-US$309. Bronchoalveolar lavage had the worst overall cost-effectiveness. We conclude that the pretest risk of active PTB was a key determinant of test utility; that the AMPLICOR assay was comparable to clinical judgement; that BAL was the least useful test; and that with increasing risks, CT scan and test combinations performed better. Further studies are needed to better define patients with intermediate risk for PTB and to directly compare the cost-effectiveness of more sensitive nucleic acid amplification tests such as the enhanced Gen Probe, CT scan and test combinations/sequences in these patients.

The rapid diagnosis of smear-negative pulmonary tuberculosis: A cost-effectiveness analysis

Objective The prompt diagnosis of smear-negative pulmonary tuberculosis (PTB) is a clinical challenge. It may be achieved by a number of tests which have varying accuracies, costs and degrees of invasiveness. The objective of this study was to compare the cost-effectiveness of clinical judgement (empirical), the Roche Cobas amplicor assay for Mycobacterium tuberculosis (amplicor), acid-fast staining of bronchoalveolar lavage specimens (BAL), nucleic acid amplification tests of bronchoalveloar lavage specimens for M. tuberculosis (BAL + NAA), computed tomography (CT) and amplicor assay followed by BAL. Methodology The range of predictive values of the various strategies were derived from published data and a new study of 441 consecutive adult patients with suspected smear-negative PTB prospectively stratified into three pretest risk groups: low, intermediate and high. The cost-effectiveness was evaluated with a decision tree model (DATATM software). Results The incidence of PTB was 5.7% (4% culture positive) for the whole group, 95% in the high-risk group, 0.9% in the low-risk group and 3.4% in the intermediate-risk group. The sensitivity of the empirical approach was 49% and of the amplicor assay was 44%. Patient outcomes were expressed as life expectancy for the base case of a 58-year-old man with a pretest probability of 5.7%. At this low pretest risk the differences in life expectancies between tests was < 0.1 years and the empirical approach incurred the lowest cost. Sensitivity analysis at increasing pretest risks showed better life expectancies (~1 years) for CT scan and test combinations than empirical and amplicor for additional costs of US$243–US$309. Bronchoalveolar lavage had the worst overall cost-effectiveness. Conclusions We conclude that the pretest risk of active PTB was a key determinant of test utility; that the AMPLICOR assay was comparable to clinical judgement; that BAL was the least useful test; and that with increasing risks, CT scan and test combinations performed better. Further studies are needed to better define patients with intermediate risk for PTB and to directly compare the cost-effectiveness of more sensitive nucleic acid amplification tests such as the enhanced Gen Probe, CT scan and test combinations/sequences in these patients.