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Originally published September, 1997
Cost resistance slows adoption of nucleic acid TB tests
When physicians need to order diagnostics for detecting tuberculosis, direct amplification tests still take a back seat to acid-fast bacillus stains. What will it take to put these rapid diagnostics in the driver's seat?
Proof of cost-effectiveness, if the presentations at a recent TB workshop held in Los Angeles are any indication. The workshop was sponsored by the Los Angeles County chapter of the American Thoracic Society.
"Los Angeles has the most up-to-date tuberculosis laboratory in the state, if not in the country," said Sydney Harvey, PhD, director of the county's public health laboratories. But in her presentation, she observed that the county's labs are using virtually all the latest technologies for confirming TB, from liquid chromatography to fluorescent staining--but not amplification.
The reasons for the county's decision have more to do with the current cost of the tests than with a lack of need for them.
For TB screening in schools and physician offices, the time-honored PPD remains the gold standard. But the consensus among health-care professionals is that for many populations this method isn't fast enough. At the Los Angeles County jail, for example, the first step in triage for jail-house bookings is that old standby, the chest x-ray, reported Paul Davidson, MD, director of tuberculosis control for the Los Angeles County Department of Health Services. Chest x-rays have the advantage that they can give a positive result in the time it takes to develop film.
But there are problems with that method, too, asserted Antonino Catanzaro, MD, professor of medicine at the University of California, San Diego, School of Medicine. "The chest x-ray is an excellent tool for identifying patients with pulmonary disease," he notes. "However, whether that disease is due to TB and whether or not the TB is active are very important questions that x-rays do not address." By contrast, direct amplification tests have shown a specificity of 100% and sensitivity of 95 and 96% in studies conducted by FDA (Am J Respir Crit Care Med, 155:1810418118, 1997).
Catanzaro concluded that tests that rapidly identify RNA and DNA panels--such as those produced by Gen-Probe (San Diego) and Roche Diagnostic Systems (Branchburg, NJ)--constitute a significant improvement.
Harvey didn't disagree. But in a study of more than 1400 samples using these nucleic acidbased tests, county investigators found that the cost of using them was prohibitive.
"At the time of the study, the cost of each PCR test was $20--just for the reagent, not the labor involved," she said. Putting these rapid tests into the county's TB screening protocol would mean "major, major cost increases." The county's conclusion: they won't be put into the protocol "until someone decides to give us a whole lot more money." --Anne Scheck
