In a report released in June, the Centers for Disease Control and Prevention (CDC) has reiterated its position that all positive screening results from both rapid oral and blood human immunodeficiency virus (HIV) assays should be confirmed with supplemental tests.

“The New York City data in this report 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,” notes the article in CDC’s June 18 Morbidity and Mortality Weekly Report. It describes unusual fluctuations in the rate of false-positive results on an oral rapid HIV test used at 10 clinics run by the New York City Department of Health and Mental Hygiene.

The CDC urges staff at other clinics to be aware of unexplained variability in the oral rapid HIV test’s false-positive rate, which has affected clinics in other jurisdictions. While the New York City clinics have stopped using the assay, the CDC continues to advocate its use in “venues where performing phlebotomy and fingersticks is impractical for screening.”

False-positive results—those that incorrectly indicate the presence of a disease when, in fact it is not present—may be a factor in some highly sensitive screening tests.  Initial tests for diseases such as HIV need to be sensitive—they need to identify as many infected people as possible and do so rapidly to allow for early treatment and to prevent the spread of the disease.  However, the trade-off for high sensitivity in a screening test is that some results that indicate a positive are, in fact, negative.  The possibility of false-positive results emphasizes the need for follow-up confirmatory tests.

The CDC has always recommended that all positive results from screening using both the oral and blood rapid tests for HIV be followed up with approved supplemental tests, such as the Western blot. While results of rapid tests are generally available within 20 minutes, results from the Western blot can take a day or more.

The article from the CDC describes two spikes in false-positive results at the New York clinics. The first occurred in November 2005. An investigation found no consistent relationship between those false-positive results and handling of the tests, storage conditions, particular lot numbers or between particular clinic sites, test operators, and patient traits. In response to the increase in false-positives, the clinics in December 2005 began administering a second rapid test on blood from finger-sticks when the oral test yielded positive results. The second spike in the rate of false positives began in November 2007 and peaked in February 2008.

The New York City clinics’ overall rate of false positives was within an acceptable range, the CDC points out. In March 2005—May 2008, the clinics administered 166,058 oral fluid rapid tests, of which 0.27% of all the tests had confirmed false positive results. That overall figure falls within the manufacturer’s limits described in the package insert. But staff at the New York City clinics had lost confidence in the oral test following the 2007-2008 increase in false positives.

Despite the reported, unexplained spikes in false positives, CDC continues to encourage the use of rapid HIV tests because they increase the number of people who are both tested and receive their results. Testing of blood is preferable because it tends to be more accurate than saliva, the article adds.

Sources

Wright, S. et al., “False Positive Oral Fluid Rapid HIV Tests – New York City, 2005-2009,” Morbidity and Mortality Weekly Report 2008. Volume 57. June 18, 2008.

Centers for Disease Control and Prevention. Quality Assurance Guidelines for Testing Using Rapid HIV Antibody Tests Waived Under the Clinical Laboratory Improvement Amendments of 1988. Available online at http://www.cdc.gov/hiv/topics/testing/resources/guidelines/qa_guide.htm. Last Reviewed July 24, 2007, 2008. Accessed July 7, 2008.

New York City Department of Healthy and Mental Hygiene. Tests and Services Manual 2005. Available online at http://www.nyc.gov/html/doh/html/labs/lab-tsmanual-hiv.shtml. Accessed on July 10, 2008.

Centers for Disease Control and Prevention. Rapid HIV Testing. “Issues with Oral Fluid Rapid Tests.” Available online at http://www.cdc.gov/hiv/topics/testing/rapid/index.htm#oral. Last reviewed on June 19, 2008. Accessed July 10, 2008.