Women who undergo Pap smear tests to screen for cervical cancer one, two or three years after having had normal test results had about the same low rate of significant cellular abnormalities, a University of California, San Francisco study has found. The finding suggests some women may not need to be screened annually, as is common practice among many U.S. physicians.
"We were interested in determining optimal screening strategies in women with recent normal Pap smears. We set out to determine if screening outcomes differed in woman who returned one, two or even three years later after a normal smear," said George Sawaya, MD, lead author and UCSF assistant professor of obstetrics, gynecology and reproductive sciences. "Our assumption was that the longer you wait, the more likely you are to have an important abnormality. We actually didn't find that. We found that it didn't make a difference if women came back for a Pap smear one, two or three years after they had a normal smear. That was against conventional wisdom."
The study results are published in the August issue of Obstetrics & Gynecology.
Over the last 50 years, the incidence of cervical cancer has declined by more than 70 percent in the U.S. The widespread availability of Pap smears--used to examine the cervix for pre invasive cells that can turn cancerous if untreated (called dysplasia)--helped to bring about the drop in cervical cancer cases. About 50 million smears are done in the U.S. each year and most are normal, according to the study.
Published guidelines suggest women with a history of normal smears may be screened less often than annually. However, it is unclear when the next smear should be performed, according to the study. Other guidelines suggest women may be screened every three years.
The study included 124,805 women at community based clinics throughout the U.S. screened for cervical cancer within three years of a normal smear. The women were examined through the National Breast and Cervical Cancer Early Detection program, sponsored by the Centers for Disease Control.
Incidence of cellular abnormalities were defined as: atypical squamous cells of undetermined significance (ASCUS), low grade squamous intraepithelial lesions (SIL), high grade SIL and a category called suggestive of squamous cell cancer. Atypical squamous cells of undetermined significance and low grade intraepithelial lesions usually do not indicate pre cancerous conditions, Sawaya said. The other two classifications are more serious signs of possible dysplasia.
The study found that clinically- important, abnormal Pap smear results are uncommon in women who wait up to three years to be screened for cervical cancer following a normal Pap smear. Over a three year period following a normal smear, the incidence of new smears interpreted as high grade SIL or worse was 66 out of 10,000 women who were 30 years old and younger; 22 out of 10,000 for women between the ages of 30 to 49; 15 out of 10,000 for those ages 50 to 64; and 10 out of 10,000 for women who were 65 years old or older.
Age adjusted incidence rates of high grade SIL or worse were similar for women screened 9-12 months (25/10,000); 13-24 months (29/10,000); and 25-36 months (33/10,000) after a normal smear.
"The bottom line is when we are making decisions on the clinical level, we need to inform patients better about the true benefits and risks of screening," Sawaya said. "If some women are advised by their physicians to extend screening intervals beyond annual, these women should realize the risk of having a bad outcome is very, very small. In fact, less screening of low-risk women may be better since the chance of false-positive testing in these women may be much higher than the chance of having significant cervical disease. We often do not inform patients of these benefits and risks since they have not been well quantified in the past."
Age adjusted incidence rates of ASCUS, the most common abnormality, did not change. But the incidence of smears interpreted as low grade SIL rose as time from the normal smear increased. These low grade abnormal smears comprised more than 95 percent of abnormalities following a normal smear, according to the study. The results are often clinically insignificant, but can prompt further tests and procedures. Such actions can cause unfounded patient anxiety, according to the study.
"There are some risks associated with screening in terms of false-positive results," Sawaya said. "In preventive medicine, we want to enhance the health of the healthy, and this is no mean feat. We need to be careful that we don't turn a healthy person into a patient because of false-positive test results." The highest rate of abnormalities occurred in the group of women 30 years old and younger. As age advanced, the incidence of all types of abnormalities decreased. For example, 1,000 women aged 65 years or older had to be screened to find one case of a highly abnormal Pap smear result after a normal smear, Sawaya said.
"We can't draw any absolute conclusions from this," he said. "But it gives us some critical pieces of the puzzle to help us determine optimal screening intervals following a normal result and identifying an age at which we may be able to recommend that screening end, given previously normal test results." Cervical cancer accounts for about 15,700 new cancer cases in the United States each year and 4,900 deaths, according to the National Institutes of Health. One of the main causes of cervical cancer is the human papillomavirus (HPV), a common sexually transmitted infection. But most women with HPV do not develop cervical cancer, Sawaya said. Half of the women who get cervical cancer in the United States have never had a Pap smear, he said.
"Rescreening women with a recent normal smear addresses but a small part of the problem of cervical cancer in the U.S.," Sawaya said.
The other study authors are Karla Kerlikowske, MD, UCSF assistant professor in residence, general internal medicine at the San Francisco Veteran's Affairs Medical Center; Nancy C. Lee, MD, from the Centers of Disease Control and Prevention; Ginny Gildengorin, PhD, UCSF statistician, department of obstetrics, gynecology and reproductive sciences; and A. Eugene Washington, MD, Msc, UCSF chair and professor of obstetrics, gynecology and reproductive sciences.
The Agency for Health Care Policy and Research funded the study.
Journal
Obstetrics and Gynecology