NYC reports spate of HIV false positives with oral test
CDC: Tests still play important role
Episodic, unexplained false positive results with an oral fluid rapid HIV test, have resulted in changed protocols in some clinics and an ongoing investigation by the Centers for Disease Control and Prevention. The test, OraQuick Advance Rapid HIV-1/2 Antibody Test (Orasure Technologies, Bethlehem, PA), can be performed on oral fluid and has broadened the settings in which HIV testing can be provided, the CDC noted.1
The New York City Department of Health and Mental Hygiene (NYC DOHMH) reported the problem, leading the CDC to underscore "the importance of routinely comparing reactive rapid test results with confirmatory Western blot test results as an essential component of quality assurance in HIV testing." Several other jurisdictions have noted clusters of false-positive oral fluid rapid HIV tests since an initial report from Minnesota in 2004, the CDC reports. "Although the causes of these clusters of false-positive tests remain unexplained, investigations are under way to determine which specific factors (e.g., test device, site, operator, or oral fluid characteristics of specific patients) might be associated with increased numbers of false-positive test results," the agency noted. " Overall, oral fluid rapid tests have performed well and make HIV testing possible in many venues where performing phlebotomy or finger sticks is impractical for screening. However, users should be aware of the unexplained variability in the rate of false-positive test results."
The incidents resulted in NYC DOHMH opting to use the finger stick rapid test while efforts are continuing to find the cause of the increase in false positive results. The first cluster of false positive tests occurred in late 2005 and subsided after several months. Another, larger increase in the incidence of false-positive oral fluid rapid test results began in late 2007 and continued through April 2008, at which time the NYC DOHMH discontinued offering this test in its STD clinics. From November 2007 to April 2008 the number of false-positive test results exceeded the number that would have been expected based on the upper limit of the manufacturer's claim. (May 2008 data, which became available after discontinuation of oral fluid testing in the STD clinics, showed that the recent increase in false-positive results had subsided), the CDC reported. From March 2005 through May 2008, the NYC DOHMH administered 160,174 oral fluid rapid tests, of which 0.27% of the tests were confirmed false positive results. Thus, overall the oral fluid rapid test performed well and within the manufacturer's limits described in the package insert. It should also be noted that during all months described in this report the test operated within the Food and Drug Administration regulations, which call for a minimum threshold of 98.0% specificity, that is, the probability that the test will be negative among patients who do not have the infection.
Oral fluid tests play an important role in HIV prevention efforts, making HIV testing possible in many venues where performing phlebotomy or fingerstick is impractical, the CDC emphasized. The agency continues to support the use of rapid oral fluid HIV tests. However, in general, FDA-approved testing with blood or serum specimens is more accurate than testing with oral fluid. In settings where blood specimens are already obtained routinely, testing with blood or serum specimens is preferred. NYC's experience demonstrates that repeating the same rapid test on fingerstick whole blood after a reactive oral fluid test result can reduce the number of false-positive test results while minimizing the number of fingerstick tests that must be performed. Even when a follow-up rapid test is performed after a reactive oral fluid test, confirmatory testing is still required, the CDC noted.
Reference
- Centers for Disease Control and Prevention. False-Positive Oral Fluid Rapid HIV Tests — New York City, 2005—2008. MMWR June 18, 2008 / 57 (Early Release);1-5 Available at: http://www.cdc.gov/mmwr.
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