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While both a positive HPV test and an abnormal result on a traditional Pap smear predicted which women would get precancerous lesions within two years of testing, the HPV test continued to predict which women were at risk for 10 to 18 years later, said study co-author Dr. Attila Lorincz, a professor of molecular epidemiology at Queen Mary University of London.
The study, which looked at nearly 20,000 women receiving routine Pap tests and HPV testing at Kaiser Permanente in Portland, Ore., is published in the July 30 issue of the Journal of Clinical Oncology.
The research does not suggest one test should replace the other, Lorincz stressed, but confirms the importance of both screenings. The main aims of the study, he said, "were to see how many extra cases of precancer can be discovered by the additional use of HPV DNA testing as compared to routine Pap testing."
The findings, he said, support recently revised guidelines suggesting that HPV testing, if negative, can allow for longer intervals between Pap testing for women over the age of 30. The findings also suggest that an alternate strategy, using HPV testing first, may work well, the researchers said.
According to American Cancer Society guidelines, the preferred screening for healthy women aged 30 to 65 is to do a Pap and HPV test every five years. Women under 30 are often infected with HPV, but clear it spontaneously. That is why the guidelines for the roles of HPV and Pap testing combined are for women aged 30 and older.
As more research is done, guidelines may change, said study co-author Dr. Andrew Glass, senior investigator at Kaiser Permanente Center for Health Research. "It is likely that HPV testing will be more and more common and may in the future replace routine Pap testing," he said.
Both the Pap test and the HPV test are done on samples of cells taken from the cervix. The Pap detects cell changes on the cervix that could become cancer if not treated appropriately.
HPV tests look for DNA, or genetic material, of the virus that can lead to precancerous changes.
In the study, women over 30 with a positive HPV at the start were more likely to have a precancer or cancer during the 10- to 18-year follow-up than those who had an abnormal Pap.
Women who tested positive for the two strains of HPV most strongly linked with cervical cancer were more likely to have precancer during the follow-up than HPV-negative women, regardless of their initial Pap test result.
Having a negative HPV more strongly predicted who would remain cancer-free than the Pap did, the investigators found.
The researchers also looked at the effect of lengthening the screening interval from three years to five in women over 30 who had negative HPV and normal Pap tests. Doing this did not increase the risk of precancer and cancers appreciably.
Both tests are commonly covered by insurance plans, sometimes requiring a co-payment. The typical cost for each test is about $40 to $70, according to Lorincz.
The study finding "confirms the fact that you don't have to do a Pap smear every year in healthy women over 30," said Dr. Mark Wakabayashi, director of the division of gynecologic oncology at the City of Hope, in Duarte, Calif.
However, he stressed the importance of checking in with a doctor. "But you have to talk to someone who understands who does need a yearly Pap," he said. "This [finding] applies just to healthy women."
The findings have nothing to do with whether you received the HPV vaccine or not, Wakabayashi said. "A lot of people are thinking if they had the vaccine they don't need a Pap smear. Not true," he said. While the HPV vaccine aims to protect against cervical cancer, screenings are still crucial.
Study leader Dr. Philip Castle, of the American Society for Clinical Pathology, does consultant work for Merck, which makes an HPV vaccine. Lorincz is a consultant for, and owns stock in, Qiagen, which makes HPV tests.
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