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    Popular Pap Test May Cause False Results

    Birth Control Pills Might Make Healthy Cells Look Abnormal

    The Appearance of HPV

    But unlike SurePath, Nuovo says the ThinPrep test uses a high-pressure system that, in filtering unwanted cells and debris, can change the physical appearance of healthy cells -- making them look as though they were infected with HPV and prompting test results suggesting the presence of atypical cells or a "false positive."

    There are nearly 100 different types of HPV, and together they infect about 24 million Americans. About one-third are transmitted through sexual contact without a condom or diaphragm and while most are harmless, some cause genital warts and 14 strains are thought to cause at least 90% of cervical cancers.

    In his study, Nuovo and his colleagues initially reviewed ThinPrep Pap tests from nearly 1,000 women taking birth control pills and compared the rates of atypical results with 1,200 women not taking oral contraceptives. The pill-takers had twice the rate of abnormal Paps test suggesting the presence of HPV.

    "It meant that either oral contraceptives put a woman at higher risk for developing abnormalities in her cervix, or that the testing method was somehow causing healthy cells to mimic virus-infected cells," he says. Going on the latter theory, he re-analyzed the data on 84 Pap smears indicating HPV. Only one in three actually had abnormal cells.

    Still the Top Test

    Still, there's no reason not to continue to use ThinPrep, say two experts not involved in the study.

    "This is an interesting observation," says William T. Creasman, MD, of the Medical University of South Carolina and a spokesman for the American College of Obstetricians and Gynecologists.

    "What it may do is get the labs to look at their data and see if, in fact, this is a phenomenon that is universal or something unique in this particular group," he tells WebMD. "If it is something beyond this group, it could have some clinical implications because then you're doing a heck of a lot more than you need to because you have a lot of false positives."

    Most women who currently have abnormal Pap tests are retested (often with the same type of smear), and/or given a specific HPV test or undergo a colposcopy, a visual examination of the cervix.

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