The purpose of drug susceptibility testing is the early detection of drug resistance. This allows the better management and treatment of patients. The early identification of drug-resistant TB cases would decrease the risk of disease and possible amplification of drug resistance. The methods currently available for drug susceptibility testing of M. tuberculosis are cheap but slow in most areas with a high incidence of TB. There is obviously a great need for fast, reliable, and inexpensive methods for drug susceptibility testing of M. tuberculosis. However, any new rapid drug susceptibility testing method must be carefully calibrated with representative isolates of M. tuberculosis in order to determine in vitro the cutoff for resistant and susceptible isolates with acceptable reproducibility.
The present meta-analysis showed that the mean values of the sensitivity and specificity of the PCR-SSCP assay were 0.79 and 0.96, respectively, and that the maximum joint sensitivity and specificity (Q value) was 0.92 and the AUC was 0.97, indicating a high level of overall accuracy. We also noted that two studies (11
) showed relatively low sensitivities (<0.70) and one study (13
) demonstrated a low specificity (<0.90) when they used PCR-SSCP assays for the rapid detection of rifampin resistance in Mycobacterium tuberculosis
The DOR is a single indicator of test accuracy that combines the data from sensitivity and specificity into a single number and the ratio of the odds of a positive test result for a patient with disease or without disease (7
). The value of a DOR ranges from 0 to infinity, with higher values indicating better discriminatory test performance (i.e., higher accuracy). In the present meta-analysis, we found that the mean DOR was 100.93, also indicating a high level of overall accuracy, and also presented both PLR and NLR as diagnostically accurate, since the SROC curve and the DOR are not easy to interpret and use in clinical practice and ratios are considered to be more clinically meaningful (4
). The PLR value was 16.10, indicating that the isolates in specimens with M. tuberculosis
have an approximately 16-fold higher chance of being rifampin resistant than the isolates in specimens with susceptible M. tuberculosis
. On the other hand, if the PCR-SSCP assay result was negative, the probability that the isolates in the sample are resistant is approximately 20%, which is not low enough to rule out resistance. These data suggest that a negative PCR-SSCP assay result should not be used alone as a justification to deny rifampin resistance in Mycobacterium tuberculosis
An exploration of the reasons for heterogeneity rather than the computation of a single summary measure is an important goal of meta-analysis (25
). In our meta-analysis, QUADAS scores were used to assess the effect of study quality on RDOR. We found significant heterogeneity for sensitivity, specificity, PLR, NLR, and DOR among the studies analyzed, although the exact mechanism responsible for the significance was unable to be explained. There were no differences between studies with or without cross-sectional, consecutive/random, and blinded designs, which may contribute to the quality of the test performance. However, PCR-SSCP assay-related technical issues in the individual studies, such as the preanalytical steps and primers used and the method of DNA quantification, may be responsible for the heterogeneity.
The meta-analysis had several strengths. First, a standard protocol was used for carrying out the systematic review (22
), including a comprehensive search strategy. Moreover, two reviewers independently carried out various stages of the systematic review process, including article selection and data extraction. Lastly, rigorous methods were used for data analysis.
The meta-analysis was limited by the relatively small number of available studies and the types of outcomes reported in the studies. Some studies presented only data on sensitivity and specificity. An obvious limitation is the lack of data on whether or not the PCR-SSCP assay has a clinical impact on patient management and treatment outcomes and how much value the assay contributes beyond that offered by conventional tests. Data on cost-effectiveness and feasibility in routine program settings are also lacking. Furthermore, there is little current evidence on how the PCR-SSCP assay may fit into existing diagnostic and treatment algorithms. Despite using publication bias analysis, the considerable heterogeneity in the results remained unexplained; further work is necessary to determine why values vary across settings. Geographic and genetic variations in the distribution of drug-resistant strains of M. tuberculosis might partially explain the present findings. Finally, the present authors excluded two studies, published in Chinese and Japanese. The exclusion of non-English-language studies, combined with potential publication bias, may have resulted in an overly optimistic estimate of the accuracy of the PCR-SSCP assay.
A notable advantage of molecular tests is their rapid turnaround time, which may have implications for patient management and transmission of drug-resistant M. tuberculosis. The turnaround time for the PCR-SSCP assay is less than 48 h, making it substantially faster than conventional drug susceptibility testing methods. An important issue that remains, however, is the affordability of molecular assays and the associated laboratory infrastructure needs in resource-constrained settings. Molecular tests, whose prices are typically higher than those of conventional tests, may be popular in resource-rich settings. However, the most resource-constrained countries tend to have the highest burden of MDR-TB cases and are the least likely to benefit from expensive technologies because of high costs and a lack of appropriate laboratory capacity.
In conclusion, the PCR-SSCP assay demonstrates a high level of overall accuracy for the detection of rifampin resistance, which is a proxy for MDR-TB. This suggests that it has good utility as a rapid screening tool, especially in settings with high rates of MDR-TB. Use of the PCR-SSCP assay is currently limited to culture isolates and direct testing of smear-positive sputum specimens, and it is not commercially available. The PCR-SSCP assay is not recommended as a replacement for conventional culture and drug susceptibility testing for the detection of rifampin resistance in M. tuberculosis.