The diagnostic accuracy of the MTBDRplus and MTBDRsl assays for drug-resistant TB detection when performed on sputum and culture isolates

Abstract

Although molecular tests for drug-resistant TB perform well on culture isolates, their accuracy using clinical samples, particularly from TB and HIV-endemic settings, requires clarification. The MTBDRplus and MTBDRsl line probe assays were evaluated in 181 sputum samples and 270 isolates from patients with culture-confirmed drug-sensitive-TB, MDR-TB, or XDR-TB. Phenotypic culture-based testing was the reference standard. Using sputum, the sensitivities for resistance was 97.7%, 95.4%, 58.9%, 61.6% for rifampicin, isoniazid, ofloxacin, and amikacin, respectively, whereas the specificities were 91.8%, 89%, 100%, and 100%, respectively. MTBDRsl sensitivity differed in smear-positive vs. smear-negative samples (79.2% vs. 20%, p < 0.0001 for ofloxacin; 72.9% vs. 37%, p = 0.0023 for amikacin) but not by HIV status. If used sequentially, MTBDRplus and MTBDRsl could rule-in XDR-TB in 78.5% (22/28) and 10.5% (2/19) of smear-positive and smear-negative samples, respectively. On culture isolates, the sensitivity for resistance to rifampicin, isoniazid, ofloxacin, and amikacin was 95.1%, 96.1%, 72.3% and 76.6%, respectively, whereas the specificities exceeded 96%. Using a sequential testing approach, rapid sputum-based diagnosis of fluoroquinolone or aminoglycoside-resistant TB is feasible only in smear-positive samples, where rule-in value is good. Further investigation is required in samples that test susceptible in order to rule-out second-line drug resistance.

Figure 1. Study plan showing the number of sputum samples tested directly using MTBDRplus (version 1) or MTBDRsl (version 1) according to patients’ diagnoses and smear-status.

The diagnosis was obtained using phenotypic liquid culture-based DST on a specimen collected at the same time as the specimen used for the line probe assays. A test is classified as positive for the Mycobacterium tuberculosis complex by the presence of the M. tb complex band (TUB), while a test is classified as negative by the absence of the M. tb complex band (TUB). Indeterminate results are those which are TUB-band positive yet are missing controls bands for gene specific loci. TB = tuberculosis; DS = drug sensitive, MDR = multi drug resistant, MDR+ =  MDR-TB but with additional resistance to OFX, KAN or INH. XDR = extensively drug resistant.

Figure 2

The testing pathway for the diagnosis of XDR-TB overall (A) or according to smear-status (B) in the clinical sputum specimens, when MTBDRplus and MTBDRsl were used sequentially. From the 234 sputum specimens tested, 47 culture-positive samples were diagnosed as XDR-TB by phenotypic DST.