Marinus Barnard¹, Heidi Albert², Gerrit Coetzee³, Richard O’Brien², and Marlein E. Bosman¹

1 National Health Laboratory Services (NHLS), Greenpoint, Cape Town, South Africa; 2 Foundation for Innovative New Diagnostics (FIND), Geneva, Switzerland; and 3 National TB Reference Laboratory, NHLS, Sandringham, Johannesburg, South Africa

Rationale: The dual challenges to tuberculosis (TB) control of HIV infection and multidrug-resistance are particularly pressing in South Africa. Conventional methods for detecting Mycobacterium tuberculosis drug resistance take weeks to months to produce results. Rapid molecular testing for drug resistance is available but has not been implemented in high-TB-burden settings.

Methods: We assessed the performance and feasibility of implementation of a commercially available molecular line-probe assay for rapid detection of rifampicin and isoniazid resistance directly on 536 consecutive smear-positive sputum specimens from patients at increased risk of multidrug-resistant (MDR) TB in a busy routine diagnostic laboratory in Cape Town, South Africa. Results were compared with conventional liquid culture and drug susceptibility testing on solid medium.

Measurements and main results: Overall, 97% of smear-positive specimens gave interpretable results within 1–2 days using the molecular assay. Sensitivity, specificity, and positive and negative predictive values were 98.9%, 99.4%, 97.9%, and 99.7%, respectively, for detection of rifampicin resistance; 94.2%, 99.7%, 99.1%, and 97.9%, respectively, for detection of isoniazid resistance; and 98.8%, 100%, 100%, and 99.7%, respectively, for detection of multidrug-resistance compared with conventional results. The assay also performed well on specimens that were contaminated on conventional culture and on smear negative, culture-positive specimens.

Conclusions: This molecular assay is a highly accurate screening tool for MDR TB, which achieves a substantial reduction in diagnostic delay. With overall performance characteristics that are superior to conventional culture and drug susceptibility testing and the possibility for high throughput with substantial cost savings, molecular testing has the potential to revolutionize MDR TB diagnosis.

AT A GLANCE COMMENTARY

Scientific Knowledge on the Subject

Molecular assays for diagnosis of drug-resistant tuberculosis are available but not widely used. There is no information on their performance in multidrug-resistant TB screening in high-burden settings. Their role in expanding global TB laboratory capacity is not established.

What this study adds to the field - such molecular assay for MDR TB diagnosis from sputum specimens can be implemented in high-burden settings.

The high accuracy, large reduction in reporting time, and high-volume capacity suggest the assay may revolutionize MDR TB diagnosis.