Monday, May 4, 2009

Rapid testing and the "window period"

Increasingly, public health officials are recognizing the problems associated with rapid HIV testing, according to an April 30 New York Times article.

Although rapid HIV tests are quick, inexpensive, and convenient (they don’t require a blood draw), the issue is the “window period”: the tests identify antibodies in the blood, not the HIV virus itself, and the body takes time (usually around 3 months, sometimes longer) to generate those antibodies. Thus, the rapid test will show up negative for HIV during a 3-month window—a time when people have especially high viral loads, according to Dr. Kenneth Mayer of Brown University. This means that infected individuals may be more likely to keep engaging in risky behaviors, and to transmit the disease. 10-50% of new HIV cases are reportedly contracted from people with acute, unidentified HIV.

The alternative? A genetic test that looks for pieces of HIV RNA, not antibodies—and so can identify the virus right after infection. The test requires a blood draw and lab work, and it takes longer to get results (a week to 10 days). It’s also a lot more expensive. However, some health departments, including San Francisco’s have started to adopt it, and it has helped them identify multiple cases of acute infection that would have otherwise slipped under the radar. As more health departments follow suit, the cost and logistical hurdles associated with the test will hopefully diminish.

Public health officials have recognized a fundamental conflict between the epidemiological facts of a disease, and the methods we’re using to control it. It’s great that they’re seeking out alternatives.

That said, I hope they look for public health solutions as well as technological ones. At least part of the issue with the rapid test can be resolved via adequate counseling and a clear explanation of the window period. Student HIV counselors at Stanford, for example, who administer rapid tests, are trained to explain the test's shortcomings to every counselee, and to ask them to return to get tested again after the window period, if necessary. Ensuring better counseling and testing practices, rather than simply promoting different testing technology, may go a long way towards reducing transmission rates.




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