Aug. 22, 2011 -- DNA tests that detect the most virulent strains of human papillomavirus, or HPV, may find more advanced precancerous cells that can lead to cervical cancer than traditional Pap smears, a new study shows.
The study, which is published in The Lancet Oncology, suggests that testing for HPV strains 16 and 18, which cause about 70% of all cervical cancer, could replace Pap tests as the primary screening tool for the disease.
“We’re not there yet, but this trial gives us more data that shows that it is reasonable to consider HPV as a standalone test,” says Therese Bevers, MD, medical director of the Cancer Prevention Center at the University of Texas M.D. Anderson Cancer Center in Houston.
The study of more than 40,000 healthy women over age 25 found that HPV testing correctly detected nearly 40% more advanced precancers than Pap smears did.
“There are still points to be worked out, but clearly this study is providing more data to say that this may be a strategy that’s reasonable to consider,” says Bevers, who was not involved in the research.
Importantly, the study also found that combining HPV and Pap testing, as many doctors do, only found about 5% more advanced precancers and cancers, but it increased the number of women who falsely tested positive by more than one-third.
“You get a 5% gain for a 35% over-referral,” says study researcher Mark H. Stoler, MD, professor of pathology and clinical gynecology at the University of Virginia Health System in Charlottesville, Va.
Those false-positive tests lead to more frequent or invasive testing to rule out cancer, more anxiety for patients, and more expense for individuals and insurance companies.
“The study suggests that if you’re really concerned about efficiency with very little penalty, then HPV testing should be the first test,” Stoler tells WebMD.
Comparing HPV to Pap Tests for Cancer Screening
The study recruited more than 47,000 women aged 25 and older from 61 clinical trial sites in 23 states.
Women were included in the study if they weren’t pregnant and hadn’t been diagnosed with low-grade cervical cell abnormalities within the last year.
All women had two swabs of cells taken from their cervix. The samples were tested for HPV viruses and the appearance of the cells was examined under a microscope.
All women who tested positive for HPV, those who had abnormal Pap smear results, and a random sample of those who had normal test results were sent to have a colposcopy, a closer examination of the cervix.
Overall, the prevalence of advanced cervical lesions among women in the study, which are considered to be precursors to cancer, was less than 2%.
Of the women diagnosed with the most advanced precancers, 92% tested positive for HPV, while 52% had abnormal Pap smear results. Among women with normal Pap smears who tested positive for HPV, the study found that those who tested positive for HPV strains 16 and 18 had significant risk of having advanced precancer.
“A woman who is HPV 16-positive with a normal Pap smear has a 10% chance of having a high-grade cancer precursor, so that’s worth her having colposcopy,” Stoler says.
The study was funded and designed by Roche, which manufactures the test for high-risk HPV strains.
Doctors Unlikely to Stop Using Pap Tests
Despite the study’s vote of confidence for HPV screening, Stoler thinks it will probably take more research and more time to convince doctors to change the way they screen for cervical cancer, Stoler says.
“It’s a matter of society and the organizations that write guidelines giving practitioners permission to do it that way,” he says. “That’s a hard thing to move.”
HPV testing with Pap testing is an option for women over 30. However Stoler says the co-test, as it is known, isn’t widely used.
Among doctors who do use it, recent surveys show that many doctors screen their low-risk patients -- those who test negative for both HPV and have normal cells on their Pap smears -- annually instead of every three years as recommended.
“This just shows that we’re grappling, as researchers and clinicians, with how to incorporate HPV testing in our cervical cancer screening,” says Elizabeth A. Poynor, MD, gynecologic oncologist and pelvic surgeon at Lenox Hill Hospital in N.Y.
“This study gives us additional information that when we look for HPV 16 or 18 or both, it is more sensitive than actually looking at the cells underneath the microscope,” Poynor says.