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Popular Pap Test May Cause False Results

Birth Control Pills Might Make Healthy Cells Look Abnormal
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WebMD Health News

July 9, 2003 -- Women who take birth control pills may be more likely to get inaccurate results indicating the presence of abnormal cells or early cancer of the cervix when their doctors use the most popular type of Pap test, suggests a new study.

In rechecking Pap smears of 84 women whose initial results were found to be abnormal with the ThinPrep smear, researchers found only a third of them had abnormal cervical cells suggesting they were infected with the human papillomavirus (HPV), a sexually transmitted disease that is a leading cause of cervical cancer. Two-thirds of the women were found not to have HPV.

All of the women evaluated in this study -- recently published in the medical journal Cancer Cytopathology -- were taking birth control pills. But the study's lead researcher tells WebMD that others may be vulnerable to get false positives from ThinPrep. The ThinPrep method of collecting samples from Pap smears has been found to be as good as, if not better than, the conventional Pap smear collection methods. And it is used by most doctors and about 80% of diagnostic laboratories.

Hormones the Culprit

"It's more common in women using birth control pills, but these false results can occur with any condition in which the woman had a predominance of certain types of female hormones -- pregnancy, taking hormone replacement therapy, and even being in certain phases of her cycle," says Gerald Nuovo, MD, professor of pathology and director at cytology at Ohio State University Medical Center.

Nuovo believes the inaccurate findings result from the nature of ThinPrep, which has gained in popularity in recent years because it is more sensitive than other tests at identifying cancerous cells. Along with the SurePath Pap test, ThinPrep is a newer, liquid-based preparation test. Rather than simply smearing the sample onto a slide, the collection device is rinsed in a vial and sent to the lab for the slide to be prepped and evaluated. .

"The problem with the older tests is that there were so much blood and inflammation in the smear that the pathologist couldn't see all the cells," Nuovo tells WebMD. "With ThinPrep and SurePath, that problem has been solved."

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.

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