Tuberculosis (TB) continues to be a leading cause of morbidity and mortality in developing countries [1
]. Global efforts for TB control are being challenged by the steady increase in drug-resistant TB, particularly multidrug resistant tuberculosis (MDR TB), defined as resistance to at least rifampicin (RIF) and isoniazid (INH). The World Health Organization (WHO) estimates that 500,000 new cases of MDR TB occur globally every year and MDR TB has been reported in 2.9% and 15.3% among the new and previously treated cases, respectively [2
MDR TB requires 18–24 months of treatment with expensive second line drugs some of which are injectable agents. The cure rate is much lower than for drug susceptible TB, only around 60% [3
]. Therefore, it is crucial that MDR TB should be detected as soon as possible, and measures implemented to effectively control its further spread.
Conventional methods for detection of MDR TB involve primary culture of specimens and isolation of Mycobacterium tuberculosis
(MTB), followed by drug susceptibility testing (DST). This process, referred to as indirect susceptibility testing has a long turn around time (TAT) of around 2 months. The TAT is longest in the TB high burden low-income countries where primary isolation and indirect DST are almost exclusively performed on solid medium. Use of liquid systems such as the BACTEC MGIT 960 system (Becton Dickinson, Sparks, Maryland, USA) has improved TAT to about 25–45 days, but liquid culture systems are in most cases not available where the need is greatest [4
Even though liquid-based indirect susceptibility tests have improved the TAT, they are still not rapid enough to allow timely decisions on patient management in case of MDR TB. More rapid TB susceptibility tests are needed, particularly in TB high burden countries. Recently, the focus has shifted to rapid direct tests in which decontaminated respiratory samples are directly inoculated in drug-free and drug-containing medium or amplified for detection of MDR TB. Some of the direct tests being studied with prospects for applicability in developing countries include the Nitrate Reductase Assay (NRA); Microscopic Observation Drug Susceptibility (MODS) assay, and more recently molecular assays such as the Genotype® MTBDR (Hain Life sciences, Nehren, Germany), and its newer version – the Genotype® MTBDRplus.
test, initially introduced as an indirect assay is performed on solid medium as for the proportion method, though liquid-based assays have recently been studied [5
]. The medium is supplemented with potassium or sodium nitrate at a concentration of 1000 mg/L to act as a growth indicator. Live M. tuberculosis
organisms possess the nitro-reductase enzyme and will reduce nitrate to nitrite, which is then detected as a pink-purple colour when a detection reagent (Griess reagent) is added to the tube [5
]. A colour change in a drug-containing tube indicates resistance. The MODS
assay is a low-technology liquid culture system performed in OADC-supplemented 7H9 broth on an ordinary tissue culture plate [10
]. A cock-tail of antibiotics – polymyxin B, amphotericin B, Nalidixic acid, trimethoprim and azlocillin (PANTA) is added to prevent growth of contaminating bacteria and fungi. Incorporation of isoniazid and rifampicin in the wells followed by inoculation of processed samples in the drug-free and drug containing wells allows direct detection of MDR TB. When M. tuberculosis
grows in the broth, characteristic cord-like structures can be seen under an inverted microscope, permitting early detection of resistance [10
]. The MODS assay has been studied on both smear positive and smear negative sputum samples with good results [11
], which is not the case with any other tests. The GenoType®MTBDR
assay is a molecular test that detects the common mutations in the rpoB
genes responsible for resistance to rifampicin and isoniazid, respectively [17
]. The test involves DNA extraction, multiplex polymerase chain reaction (PCR), solid phase reverse hybridization and detection of the resistance mutations [18
]. The Genotype®
assay detects additional mutations in the rpoB
gene and also in the inhA
gene promoter region, giving a higher sensitivity in resistance detection [18
Published studies have evaluated the performance of direct testing with the above mentioned tests. However, the data is spread in many different journals, which makes it difficult to fully understand the performance of direct testing, thereby delaying decisions on adoption of this approach for prompt detection of MDR TB. In this study, available data from individual study reports on direct testing with the NRA, MODS, Genotype® MTBDR and Genotype® MTBDRplus was pooled and analysed for sensitivity, specificity and time to results of direct testing against conventional indirect susceptibility testing in detection of MDR TB. The results of this meta-analysis are intended to guide TB control programmes in TB high burden countries to select for further operational study, highly sensitive and specific rapid tests to identify MDR TB.