Improving detection of drug-resistant tuberculosis (TB by use of novel phenotypic and genotypic tests

In 2011 the estimates of new cases of TB were 8.7 million, with 1.4 million deaths from TB, 1 million in non- HIV patients and 430 000 in co-infected patients. Multidrug resistant tuberculosis (MDR-TB), resistance to the two most powerful anti-TB drugs, INH and RMP, is recognized as a global epidemic. When this resistance pattern is combined with resistance to fluoroquinolones (such as ofloxacin or moxifloxacin) and to injectable second-line drugs (amikacin, capreomycin or kanamycin), the isolate is considered extremely drug resistant (XDR). WHO reports that among all TB cases notified in 2011 in the world, between 220,000 and 400,000 are MDR-TB cases. The WHO global TB report of 2012 lists Georgia among the 27 high TB burden countries, with 11% of all new TB cases diagnosed with MDR and 32% of previously treated cases1. WHO recognizes that, thanks to National Tuberculosis Programs (NTPs) efforts, case detection is improving, yet the inadequacy of conventional laboratory capacity persists2. Extensive phenotypic DST to test susceptibility to anti-tuberculosis drug is recommended but lack of accessible and rapid techniques in low-income settings contributes to treating patients with ineffective treatment regimens as resistance to the prescribed drugs is unknown; these patients fail treatment and become chronically ill with TB, or die. Development of affordable and reliable techniques is urgently needed to support the design of effective treatment. Intrinsic difficulties to test drugs included in treatment regimens contributes to increasing resistance. WHO recommends use of effective drugs like ethambutol (EMB) in combination with streptomycin, isoniazid, rifampicin and pyrazinamide in the management of tuberculosis (TB) patients. DST for EMB however often shows discrepancies3,4, making the interpretation of the test difficult, and raising doubt on its utility. On the other hand, the embB mutation, found in 70% of isolates with EMB resistance, may cause a broader spectrum of resistance, beyond EMB resistance5, which justifies further investigation into a possible relation between EMB resistance and MDR resistance6,7. In December 2010 WHO endorsed Xpert® MTB/RIF, an automated nucleic acid amplification technique (NAAT) to be used in TB endemic countries to speed detection of new cases and diagnosis of MDR TB8. In demonstration studies9 this test was found to highly increase case detection and to provide rapid screening of RMP resistance; however in low-MDR prevalence settings these results require confirmation either with Line Probe Assay (LPA) or with conventional DST10. In MSF settings Xpert® MTB/RIF has shown to increase detection of new TB cases and to provide rapid results for RMP resistance11, but still access to culture laboratories is a requirement of all TB programs. LPA available on the market to detect resistance to drugs other than INH and RMP, such as the MTBDRsl test from Hain lifesciences, can detect mutations associated with ethambutol, fluoroquinolones and aminoglycosides resistance. This assay provides rapid results but does not consistently perform directly on samples, requiring use of isolates obtained from cultures. Evidence of performance is too weak for the test to be endorsed by WHO12. PCR-based techniques do not distinguish between dead or viable organisms, and do not eliminate the need for conventional culture, both for patient monitoring and to provide isolates for second-line drug testing by culture or LPA. Most common phenotypical tests recommended are based on use of solid media like Lowenstein-Jensen (LJ) and 7H11 agar based medium, and liquid media, such as Mycobacteria Growth Indicator Tube BACTEC™ MGIT™ 960 (MGIT) liquid medium. While solid media are cheaper but slower, with DST results available after 3 to 4 weeks, the MGIT method gives DST results within maximum 12 days but is still rather expensive for low resources settings and has more complex logistical requirements13. Cultures and conventional phenotypic tests are still considered the gold standard, but long time to results, high cost, considerable biosafety hazard, cumbersome procedures, high contamination when applied to samples processed after long shipment14,15 and unclear resistance mechanisms for key drugs like EMB limit their application. Non-commercial methods like Thin Layer Agar (TLA), based on use of 7H11, showed culture performance similar to MGIT16,17, and comparable DST performance in terms of sensitivity and specificity18, with shorter turnaround time, with lower costs and limited logistical requirements, but literature available on this application for direct DST is still limited19. Rapid tests for prompt detection of resistance to anti-tuberculosis drugs is of great importance for control of TB, especially in settings with a high prevalence of MDRTB. In this proposal, we therefore focus on the optimized use of novel low cost phenotypic and genotypic tests.

Trefwoorden:B780-tropische-geneeskunde