Quick Test for Rifampin-Resistant TB Shows Promise

An automated assay designed for use in Third World regions rapidly and accurately detected Mycobacterium tuberculosis infection and resistance to rifampin, according to a report published online Sept. 1 in the New England Journal of Medicine.

In a multicenter, prospective trial in South Africa, Peru, India, and Azerbaijan involving 1,730 patients suspected of having TB, the Xpert MTB/RIF correctly identified 72% of patients whose sputum smears were negative, as well as 98% of those with positive smears. It also correctly identified 98% of rifampin-resistant bacteria and 98% of rifampin-sensitive bacteria, said Dr. Catharina C. Boehme of the Foundation for Innovative New Diagnostics (FIND), Geneva, and her associates.

“Only a small fraction” of patients worldwide with drug-resistant TB currently has access to sufficiently sensitive diagnostic testing and drug-susceptibility testing, Dr. Boehme noted, because of the complex technologies required for mycobacterial culture and nucleic-acid amplification (N. Engl. J. Med. 2010 Sept. 1 [doi: 10.1056/NEJMoa0907847]).

“Globally, ineffective tuberculosis detection and the rise of multidrug resistance and extensively drug-resistant TB have led to calls for dramatic expansion of culture capability and drug-susceptibility testing in countries in which the disease is endemic,” Dr. Boehme and her colleagues noted. “Unfortunately, the infrastructure and trained personnel required for such testing are not available except in a limited number of reference centers, and results of testing are often not available for at least 4 months, which dramatically reduces its clinical utility.”.

FIND developed the new assay to address those needs. FIND also designed, supervised, and sponsored the study evaluating the assay’s performance.

The Xpert MTB/RIF kit includes a disposable plastic cartridge that contains all the reagents needed for bacterial analysis, nucleic acid extraction, PCR amplification, and amplicon detection. The only manual step is the “nonprecise” addition of a bactericidal buffer to sputum before transferring the sample to the cartridge. Because the cartridge is never reopened, there is little chance of amplicon contamination, the investigators noted. In addition, the sputum is inactivated at the same time it is liquified, thus making a biosafety cabinet unnecessary.

The cartridge is then inserted into the GeneXpert device, which delivers test results within 2 hours. Relatively unskilled health care workers at all the study locations became proficient in the assay’s use after a brief training session. Recent data from a separate study confirm that the assay generates no infectious aerosols, which obviates the need for laboratories equipped for advanced biosafety.

Of the 1,462 patients (4,386 sputum samples) assessed, 567 patients had smear-positive and culture-positive TB; 174 had smear-negative but culture-positive TB; 105 had clinically defined but smear-negative, culture-negative TB; and 616 had no clinical, smear, or culture evidence of TB. The remaining 268 patients were excluded from the study for a variety of reasons, including 103 who had an inadequate number of sputum samples and 10 who had an inadequate volume of sputum samples.

Overall sensitivity of the device among patients with culture-positive TB was 97.6%, with no significant variation in performance across the study sites. That suggests that the study findings “are likely to be widely applicable,” Dr. Boehme and her associates said.

Sensitivity was 99.8% for smear-positive and culture-positive cases, and 90.2% for smear-negative but culture-positive cases. The assay was specific in 604 of the 609 patients who proved not to have TB (99.2%).

In addition, “the MTB/RIF test correctly detected rifampin resistance in 209 of 211 patients (99.1% sensitivity)” and correctly identified rifampin susceptibility in all 506 patients who had it (100% specificity).

“In view of the low sensitivity of smear microscopy for the diagnosis of TB in patients with HIV infection, the increased sensitivity of the MTB/RIF test – notably, among patients with smear-negative tuberculosis – at the two South African sites with 60% to 80% prevalence of HIV infection is encouraging,” the researchers noted.

It is not yet known whether the results can be replicated “in microscopy centers, health posts, and other point-of-treatment settings where temperature and electricity supply will be more variable and training issues will be more relevant,” the investigators cautioned.

“Large-scale projects to show the feasibility and effect of MTB/RIF testing at such sites are under way,” they added.

The study was designed and supervised by the sponsor (and maker) of the Xpert MTB/RIF, FIND, with additional development support provided by the National Institutes of Health, Cepheid, and the Bill and Melinda Gates Foundation. The investigators reported no additional disclosures.

In an editorial accompanying Dr. Boehme’s report, Dr. Peter M. Small and Dr. Madhukar Pai said that the Xpert MTB/RIF assay has the potential to revolutionize the diagnosis of tuberculosis and has several critical advantages over conventional nucleic acid amplification tests.