The new studies, published Friday by the U.S. Centers for Disease Control and Prevention, used data from phone surveys that asked women about their screening habits.
Those surveys were conducted between 2000 and 2010, before the 2012 guidelines. But the data suggest that legions of women (and their doctors) are clinging to outdated advice:
In 2010, almost 60 percent of women with hysterectomies reported a Pap smear within three years, down from 73 percent in 2000. The researchers estimated 22 million such women had unnecessary screening over the period.
In 2010, almost half of women 18 to 21 said they had been tested, down from three-quarters in 2000. This age group doesn't need screening.
Even though a three-year screening interval is advised for women 22 to 30, annual tests remain the norm. In 2010, 67 percent had been tested within a year, and 15 percent had been tested within two years.
Will the Affordable Care Act, which requires insurers to cover cervical cancer screening with no out-of-pocket costs, add to the problem of overscreening? The CDC researchers could say only that new public health campaigns are focused on changing habits.
"One of the best things we can do is educate providers and women to understand that more is not always better," said CDC epidemiologist Meg Watson. "There are some harms to unnecessary screening, and the resources would be better directed to women who really need it."
Experts stress that Pap smears, introduced in the 1950s, are still important, and that some women, notably those without insurance or regular doctors, are not getting tested often enough.
The simple test - in which cells are examined for precancerous changes under a microscope - enables curative treatment before the changes progress to cancer. Cervical cancer deaths have fallen by 70 percent since the 1950s; now, about 13,000 women are diagnosed and 4,000 die annually.
But scientists have discovered a lot about cervical cancer since the advent of screening. The disease is caused by certain strains of the sexually transmitted human papilloma virus (HPV). DNA tests can now detect the virus.
Indeed, for women over 30 who have negative Pap results, the recommended screening strategy is "co-testing" with the Pap and HPV tests once every five years. (Two vaccines to prevent HPV infection are also available, but these are too new to influence screening practices.)
Research has shown that most precancerous changes go away without treatment, and progression to cancer usually takes years or decades. And, contrary to past thinking, women with total hysterectomies can't use Pap smears of vaginal cells to monitor for vaginal cancer.
It has also become clear that treatment can affect fertility and pregnancy.
"Cone biopsies [of the cervix] are a serious issue, especially in younger women," said Virginia Moyer, chair of the U.S. Preventive Services Task Force, an issuer of the 2012 screening guidelines. "It's one of the reasons we don't want to overdo it."
Contact Marie McCullough at 215-854-2720 or firstname.lastname@example.org.