Although most women are accustomed to having Pap smears to test for cervical cancer -- a disease that strikes 12,000 American women each year, according to the U.S. Centers for Disease Control and Prevention -- a new study found that the HPV test may be a more effective way to catch the often-preventable cancer before it progresses.
Most cervical cancers are caused by a particular strains of the Human papillomavirus, or HPV. The test for detecting HPV is simpler than a Pap smear and, the study published in the Journal of the American Medical Association found, it may be better at catching the earliest cervical cancer stages.
Pap smears involve scraping cells from the cervix and examining them for cancerous changes, also known as "cytology" testing. They detect only around 55 percent of true cases, according to a study by the Canadian Cervical Cancer Screening Trial Study Group, and requires a special process using a liquid medium that takes several days.
The HPV test, which has not traditionally been offered to all women, might be simpler and more accurate. The test uses a similar technique, but with an easier process of using vaginal swabs, and relies on detecting the HPV virus via DNA. According to the same trial study group, the test finds 94.6 percent of suspected cervical cancer cases.
In the U.S., using both the Pap smear and the HPV test, or co-testing, is the gold standard recommended by the U.S. Preventative Services Task Force. They recommend that women between 21 and 65 should have a Pap smear every three years, but that women aged 30 to 65 may choose to have both the Pap smear and HPV testing together every five years.
This study suggests that the HPV test alone may be more effective than the Pap smear alone at predicting the development of cervical cancer. The findings, using research from the British Columbia Centers for Disease Control in Vancouver, Canada, are based on a large, randomized clinical trial of 19,000 women of varying ages, races and socio-economic statuses, comparing HPV testing with a Pap smear.
Half of the women in the study were randomized to get the HPV test and half to the standard Pap smear screening alone. The women who tested negative with the HPV test returned four years later. Those who tested negative with a Pap smear returned two years later and, if they tested negative again, another two years later. At the end of four years, both groups received both tests.
The HPV test was more likely to pick up pre-cancerous cervical lesions earlier in the study, so the women in this group were significantly less likely to develop invasive neoplasia, or cervical cancer.
However, because all women who were HPV test negative in the study also received a Pap smear, additional cases of early cancer were detected.
"Here in B.C. we have an organized screening program, but we still miss early cervical cancer in women with apparently normal Pap smears," lead author Dr. Gina Ogilvie, a family physician and senior public health scientist at the BCCDC, told ABC News. "We felt there must be a better way."
Of note, the Canadian Task Force on Preventative Health Care differs from the U.S. task force -- it recommends Pap smear screening every three years between ages 30 and 69, citing weak evidence for screening women ages 25 to 29. The Canadian task force does note that HPV testing may be recommended once more research is available. This could be that research, and while impressed with the results, Ogilvie wasn’t surprised.
"We knew from previous cross-sectional data that HPV does detect more pre-cancerous lesion earlier. If the HPV test is negative women are significantly less likely to develop pre-cancer during the following four years," Ogilvie told ABC News.
One limitation of the study was that it was done in a specific area of Canada and the findings may not be generalized to other populations.
"This was a very well-done study. But one issue is that it was centered around one perfect system in British Columbia, where they had one place to send all of the Pap smears and HPV testing. They also had good follow-up of each research participant. In most places that’s not the case," Dr. Kathleen Schmeler, associate professor in gynecological oncology and reproductive medicine at the University of Texas MD Anderson Cancer Center, who was not involved in the study, told ABC news.
Schmeler often works in Latin America where, in countries like El Salvador, cervical cancer is one of the most common cancers in women primarily due to poor screening programs.
Schmeler points to the importance of the HPV vaccine as a crucial preventative measure, but also to the debate between different medical organizations' screening recommendations.
"A few months ago, when the U.S. task force released draft guidelines to consider only HPV testing, there was lots of controversy," Schmeler told ABC News. "The ASCCP [American Society for Colposcopy and Cervical Pathology] pointed out that just doing the HPV testing would miss some people, and so they advocated for co-testing."
Because all the women in the study who testing negative for HPV were given Pap smears at the end of the study, the results can't determine how one test alone compares to co-testing.
"We really can’t answer the question around whether co-testing is better than either alone. We know that the HPV test missed some early cancerous lesions in some women," Dr. Mark Spitzer, Medical Director of the Center for Colposcopy and past president of the ASCCP, told ABC News. "In fact, at the conclusion of the study, all HPV-negative women were tested with the Pap smear, which led to finding additional cases of pre-cancerous lesions."
He said co-testing with both Pap smear and HPV remains and effective approach for cervical cancer screening.
"This supports the small, but significant benefit of co-testing. So while the HPV test is more sensitive of a test, it isn’t perfect and missed some women with early signs of cervical cancer. Most women and doctors are not willing to give up this additional impact," he added. "If you’re one of those people, for instance, it’s very significant.”
Because the HPV vaccine that is recommended for adolescents is still limited in its use, testing for the presence of the HPV virus, in general, will continue to be a crucial part of cervical cancer prevention.
"At the moment, at most, we may be vaccinating at most 30 to 40 percent of girls. Even if we could magically vaccinate 100 percent today, it would take 30 years for them to reach the age of 40, and to see a meaningful decrease in cervical cancer," Spitzer added. "In my opinion, we should not change screening guidance today based on an unknown future that may not be realized in this country for decades."
Researchers in the study said one of the next steps is to look into whether the additional cases of cervical cancer that could be found with HPV testing would justify the cost of co-testing -- or whether HPV testing alone would be more cost effective.
Even the most perfect tests can also be limited by the fact that not all women are receiving regular and timely screening.
"Whether it’s co-testing or choosing one test over another, it’s really important to think about the system in which cervical cancer screening is introduced, and that women are called back and followed-up for an abnormal test, whether it be a Pap smear or an HPV test," Ogilvie said.
Amitha Kalaichandran, M.D., is a Pediatrics resident working in the ABC News Medical Unit.