Oct. 18 (HealthDay News) -- A test to detect human papillomavirus (HPV) -- which causes most cervical cancers -- was far better than the standard Pap smear at catching malignancies, two new studies show.

What's unclear, however, is whether or not the HPV test should take the place of Pap testing, because it can have a slightly higher false positive rate, and it's not as effective in younger women.

"There are still many questions related to the use of HPV testing as the primary screening test," said Debbie Saslow, director of breast and gynecologic cancer for the American Cancer Society. "At this time, HPV testing alone (without a Pap test) is not FDA-approved as a primary screening test, so that approach is not recommended by the American Cancer Society."

But, Saslow pointed out, "The most important message remains unchanged: Women should be screened using one of three options: a conventional Pap test, liquid Pap or Pap plus the HPV test. For women who have access to HPV testing, the HPV test offers added benefits over [Pap tests] alone."

Pap tests have been the standard screening test for cervical cancer for the past 60 years. More recently, the liquid-based or thin-smear Pap test was developed, and it was initially thought that this newer technology would offer significant advantages over the traditional Pap test. However, studies haven't conclusively found a significant benefit to the newer test. Because most cervical cancers are caused by HPV, the test to detect HPV infection is also an option for screening in addition to the Pap test.

The most recent research comparing these screening tests appears in the Oct. 18 issue of the New England Journal of Medicine. The first study, done in Canada, compares the use of conventional Pap alone to the use of the HPV test alone.

The study included more than 10,000 women between 30 and 69 years old who were randomly assigned to one of the tests. The HPV test detected 94.6 percent of the abnormal growths that can lead to cervical cancer, while the Pap test found only 55.4 percent of these growths. Women who had an abnormal result were referred for further testing. If both Pap and HPV were used together, 100 percent of the abnormal growths were found. The false-positive rate for HPV was only 2.7 percent higher than the rate for Pap smears, according to the study.

"Even though Pap has worked, we have a test that's better than Pap, and in the coming years, we can switch to HPV and possibly screen every three or maybe even every five years and get the same level of protection," said study author Dr. Marie-Helene Mayrand, an assistant professor of obstetrics and gynecology and social and preventive medicine at the University of Montreal. Mayrand was a doctoral candidate at McGill University in Montreal at the time of the study.

The second study, done in Sweden, included more than 12,000 women in their 30s who were randomized to receive either a conventional Pap test alone or an HPV test and a Pap test.

The researchers found abnormal cervical changes in 51 percent more women who had both tests than in the Pap group alone. In subsequent screenings, about 40 percent fewer precancerous lesions were found in the women who'd been screened with both tests compared to women who'd had the Pap test alone, which suggests that overdiagnosis from the HPV test is not as big an issue as some experts had feared.

"Adding HPV testing to Pap testing in routine cervical screening gives a longer protection against the severe precursor lesions of cervical cancer compared to just Pap testing," said study co-author Dr. Joakim Dillner, a professor of virology at Lund University and Malmo University Hospital in Malmo, Sweden.

"In women 30 years of age and older, it is likely to eventually replace the Pap test. The major benefit of the HPV test is that it, compared to Pap testing, enables lengthening the screening interval substantially," Dillner said.

Several questions remain, however. One is how the HPV test compares to the liquid-based Pap test, as neither of these studies used the newer test. And, Saslow pointed out, an emerging group of young women will have been vaccinated against HPV, and it's not clear how that might affect the reliability of the HPV test in the future.

SOURCES: Joakim Dillner, M.D., Ph.D., professor, virology, Lund University and Malmo University Hospital, Malmo, Sweden; Marie-Helene Mayrand, M.D., assistant professor, obstetrics and gynecology and social and preventive medicine, University of Montreal; Debbie Saslow, Ph.D., director, breast and gynecologic cancer, American Cancer Society, Atlanta; Oct. 18, 2007, New England Journal of Medicine