Marinus Barnard performing MTBDRplus testing at NHLS TB laboratory in Cape Town

In response to the expanding epidemic of multidrug-resistant tuberculosis (MDR TB) and the more serious form of extensively drug resistant (XDR) TB, FIND has committed to undertake large-scale demonstration projects of new technologies for the rapid screening of MDR TB. The largest of these is underway in South Africa where the Hain GenoType® MTBDRplus test is being used to screen 20,000 MDR TB at-risk patients over the course of one year.

The Hain test is a multiplex polymerase chain reaction line-probe assay that detects M. tuberculosis complex and genetic mutations associated with isoniazid and rifampin resistance in one day. Three published studies of an earlier version of this test indicated good performance when applied to AFB smear-positive sputum specimens. The second generation assay includes a new amplification control to increase test sensitivity and a second probe for isoniazid resistance.

Because the enhanced test had not been extensively evaluated, FIND undertook validation studies in two laboratories in South Africa. The largest of these was conducted in the National Health Laboratory Service (NHLS) laboratory in Cape Town, a large and busy TB laboratory with no prior experience in molecular testing. Over the course of one month, over 500 smear-positive sputum specimens submitted for MGIT culture and drug susceptibility testing (DST) on 7H10 medium were tested by the Hain assay.

Approximately 97% of Hain tests had interpretable results, i.e., detecting the presence of M. tuberculosis complex, and valid results for isoniazid and rifampin susceptibility or resistance. Over 10% of MGIT cultures were contaminated, so that conventional DST results were not available. Compared to conventional DST, the sensitivity, specificity, and positive and negative predictive values were 99%, 100%, 100% and 100%. Test sensitivity and accuracy for a smaller number of smear-negative specimens was also good. With results available within one or two days, estimated turn-around time (TAT) from specimen collection to reporting results was under seven days. This compares to a TAT of two to three months for conventional testing.

As concluded in a manuscript recently published in the American Journal of Respiratory and Critical Care Medicine, the overall performance of this assay was superior to conventional testing. With the potential for high throughput at a lower cost than conventional testing, this assay has the potential to revolutionize the diagnosis of MDR TB. Furthermore, new specimen processing procedures currently under evaluation may both increase test sensitivity and facilitate specimen transport.

Following validation studies in three other NHLS laboratories with similar results, the assay has moved into demonstration. To date approximately 10,000 patients have been screened. Similar projects are being planned for Turkey, India, Vietnam, China, Thailand and Uganda.

Data on test performance, patient impact, and cost-effectiveness are being compiled for presentation to a WHO expert review group in March 2008. It is expected that this group will develop guidance on the use of molecular line-probe assays for MDR TB screening. This guidance would then be reviewed by the WHO Scientific and Technical Advisory Group for Tuberculosis (STAG TB) at its meeting in June.

Abstract brief