McGill University researchers who analyzed 18 studies involving more than 7,000 people released findings today showing the GeneXpert rapid diagnostic test for tuberculosis detected the disease among 67 percent of samples in which it had been missed by smear microscopy, the most common diagnostic tool in low-resource but high burden countries.
While both tests require a sample of sputum, the GeneXpert can detect the bacteria at significantly lower concentrations than necessary for the disease to be detected by a microscope. In addition, while smear microscopy can’t detect drug resistance, the analysis found the GeneXpert detected resistance to the antituberculosis drug rifampicin in 141, or 94 percent, of 150 patients who were determined in subsequent laboratory tests to be resistant to the drug.
The GeneXpert diagnostic test for TB was endorsed by the World Health Organization in 2010, and in addition to detecting bacteria missed by microscope examination, it delivers results within a couple of hours and requires relatively little expertise.
The findings of the analysis are consistent with what was known of the test’s effectiveness when it was approved by the WHO, but represent a larger and consistent body of evidence of the test’s advantages, said Dr. Madhukar Pai, a McGill professor and TB researcher who worked on the study. The evidence is important because while the cost of using the tool has recently dropped, with a reduction from more than $18 to under $10 per single use cartridge used for each test in some countries, the diagnostic machine, which itself started with a cost of more than $17,000, is beyond the reach of many of the settings where it is needed most, despite the advantages offered by its accuracy.
The analysis does not look at cost effectiveness, but in the time since the test was introduced, at least a dozen studies have looked at the savings generated by more accurate and timely diagnoses of tuberculosis and drug-resistant strains, Pai said.
The analysis also did not examine data on the test’s relative effectiveness in detecting extra-pulmonary or pediatric tuberculosis, but analyses of those are currently underway.
“Quite a few countries are in the process of decision making on scaling up use of the test,” Pai said. The analysis is likely to add more weight to their considerations than individual studies, he said, adding, “They still have to look at the missing piece, which is money.”
“It’s accurate, it’s better than smear microscopy,” Dr. Pai said, “but that doesn’t mean countries can afford it.”