Abstract:
BACKGROUND : A major challenge faced by countries with a high burden of tuberculosis (TB) is early detection
especially in individuals with paucibacillary disease which is common in HIV endemic settings.
Remarkable efforts have been made globally to accelerate the development and expansion of new diagnostic
technologies that allow better and earlier diagnosis of active tuberculosis particularly directly from clinical specimens
with a few commercial options available. These include GenoType MTBDRplus Version 2.0 (Hain Lifescience), Xpert®
MTB/RIF (Cepheid) and Anyplex™ plus MTB/NTM/DR-TB Real-time detection (Seegene).
We evaluated the diagnostic performance of these three commercial molecular assays for the detection of Mycobacterium
tuberculosis complex from clinical specimens in a high TB-HIV-burden setting.
METHODS : This was a retrospective laboratory-based study using stored remnant sediments from clinical specimens of
presumptive pulmonary TB cases. A stratified sample of smear positive TB, smear negative TB and TB culture negatives
was included. All the samples were tested on the three molecular assays following the manufacturers’ instructions; except
for Anyplex™plus, for which DNA extraction was performed using the NucliSENS® easyMAG® platform (bioMerieux).
Samples were also processed for liquid TB culture and time-to-culture positivity was recorded.
RESULTS : Of the 90 sediments processed, 81 were analyzable across all three systems. The overall sensitivity was highest
for Xpert® MTB/RIF (89.1 %) followed by GenoType MTBDRplus (70.9 %) and Anyplex™ plus (65.5 %). The specificity and
sensitivity in smear positive cases was comparable across all systems. There was a significant difference in sensitivity
between Xpert® MTB/RIF and the other two assays for smear-negative cases (P < 0.05). The performance in cases where
the time-to-culture positivity was ≥20 days was also significantly poorer for both Anyplex™ plus and GenoType
MTBDRplus compared to Xpert® MTB/RIF (P < 0.05). Xpert® MTB/RIF achieved 100 % specificity, while Anyplex™
plus and GenoType MTBDRplus achieved 96.2 and 92.3 % respectively.
CONCLUSION : The Xpert® MTB/RIF was superior to the other two assays for the detection of TB in smear negative
specimens notably when bacterial loads are very low in sputum. It is important that studies reporting on test
performance stratify their results by time-to-culture positivity to accurately assess clinical performance especially
in high HIV settings.
