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CHICAGO -- Screening for cervical cancer using Pap or HPV testing can safely take place every three years for women over 30, a researcher reported.

And the more recent test for human papillomavirus (HPV) may be more accurate than the standard Pap smear in determining cancer risk, according to Hormuzd Katki, PhD, of the National Cancer Institute in Bethesda, Md.

The findings, based on medical records of more than 300,000 women, are a real-world confirmation of what has been seen in studies, Katki told reporters in an advance press conference for the June American Society for Clinical Oncology (ASCO). Katki's group is scheduled to present the results at ASCO 2011.

"We already have a lot of clinical trial and research cohort evidence about the potential value of co-testing," he told 乌鸦传媒. "What had been missing was real-life experience and that's what we have provided."

Indeed, the study provides "very solid data to make us comfortable" with recommendations for a three-year gap between tests, according to ASCO president George Sledge Jr., MD, of Indiana University in Indianapolis.

"The fascination of this study is that it is a very large real-life experience," Sledge told 乌鸦传媒.

Current guidelines from the American Cancer Society say women over 30 who have had three normal Pap test results in a row can get screened every two to three years.

Women older than 30 may also get screened every three years with either the conventional or liquid-based Pap test or the HPV test, the guidelines say.

However, Katki noted, such co-testing has not been widely adopted, largely because many physicians and women are not sure about the safety of extending testing intervals for more than one year.

But changes in recommendations are likely over the next few years, Sledge said.

The study involved medical records of 331,818 eligible women, 30 years or older, enrolled in co-testing programs of Kaiser Permanente Northern California from 2003 through 2005.

To estimate their cancer risk, they were categorized by the result of their Pap and HPV tests at enrollment and followed through 2009, Katki told reporters.

Analysis showed:

• For all women with a normal Pap test, there were 7.5 cervical cancers per 100,000 woman/years.
• For all women who were HPV-negative, the rate was 3.8 cervical cancers per woman/years.
• For women who were HPV-negative and also had a normal Pap test, the rate was 3.2 cervical cancers per 100,000 woman/years.

Those results suggest that HPV testing may be more accurate than the Pap smear, Katki said.

The study also found that HPV testing identified more women at high risk for cervical cancer than Pap tests, he reported.

Women who were HPV-positive at enrollment had a five-year risk of cancer or a pre-cancerous lesion of 7.6%, while the risk for women who had an abnormal Pap smear was 4.7%, he reported.

The converse was also true -- a negative HPV test was associated with a lower risk than a normal Pap smear, he said.

But that doesn't mean a Pap smear is useless, he said. Combining the two added significant information if both were positive.

Specifically, women with a positive HPV test and a normal Pap test had a five-year risk of cancer or pre-cancer of 6%, he said. But if both tests were positive, the risk was 12%.

Outside experts said the findings are a useful confirmation of previous evidence, but suggested they will not lead to a rapid change in clinical practice, largely because women and their doctors are used to annual Pap smears.

"This is not entirely new, but it is nice to get confirmation," according to Ranit Mishori, MD, of Georgetown University School of Medicine in Washington, DC.

Mishori told 乌鸦传媒/ABC News in an email, "many providers have a hard time changing habitual practice and adopting new evidence and practice guidelines."

Most women still undergo annual screening, according to Brent DuBeshter, MD, of the University of Rochester Medical Center in Rochester, N.Y.

But it's mainly out of habit, he said in an email to 乌鸦传媒/ABC News. "This was the recommendation for so many years that many physicians and patients aren't comfortable with the new guidelines that call for screening every three years in those at low risk for cervical cancer," DuBeshter argued.

On the other hand, the conclusions of the study were criticized by Diane Harper, MD, of the University of Missouri Kansas City.

In an email to 乌鸦传媒/ABC News, Harper said the study doesn't take into account a high rate of false positive tests and applies to a specific population of women over 30 who have already been screened intensively since their sexual debut.

In those women, she said, the prevalence of pre-cancerous lesions and cancer will be low.

Harper noted that the Netherlands is planning a new testing program that will start at age 25 and rely primarily on HPV tests, with the test repeated every five years.

"This is based on excellent data and is a solid way to care for a population of women without causing too many false positives," Harper argued.

This article was developed in collaboration with ABC News.