Taking some of the confusion out of screening strategies
By William A. Check, PhD
CHICAGO—Primary care physicians are not using screening tests to their full potential, according to experts in preventive medicine. At a recent Primary Medicine Today conference, speakers offered suggestions for how doctors can make sense of some of the more confusing aspects of current prevention strategies.
David L. Hahn, MD, medical director of the Dean Clinic in Madison, Wis., said he thinks most of physicians' reluctance to use screening tests has to do with attitude. "We physicians live in a 'crisis clinic' mentality," he said. "We are taught to deliver acute care and to respond to illness and immediate symptoms."
As an illustration, Dr. Hahn described how in 1992 the Dean Clinic tried to increase the use of 10 preventive services ranging from screening for high blood pressure to Pap smears and mammography. While physicians in the group voiced support for the plan, only a handful actually increased their use of these services. Pap smears were performed for 78% of eligible patients, but mammography was done for only 60% of women over age 50. And while cholesterol was measured in 80% of patients, fecal occult blood testing (FOBT) was offered during only 36% of patient visits.
Physicians may also be reluctant to use preventive services because of the plethora of sometimes contradictory screening guidelines, Dr. Hahn said. To make sense of the guidelines and tests available, he suggested that clinicians consider the following six criteria:
- Screening for a condition must affect the extent or quality of life.
- Acceptable methods of treatment must be available.
- The condition must be detectable before it becomes symptomatic, and treatment must be able to significantly reduce morbidity or mortality.
- Treatment in the asymptomatic phase must yield a therapeutic result superior to that obtained by waiting until symptoms appear.
- Tests must be available to detect the condition while it is asymptomatic and be acceptable to patients at reasonable cost—although these terms often mean different things to different patients.
- The incidence of the condition must justify the cost of screening.
Dr. Hahn suggested that primary care physicians base their decisions on reports from the U.S. Preventive Services Task Force (USPSTF), which incorporate these criteria and grade the strength of evidence for each practice evaluated. (For a 1996 report, contact the Government Printing Office at 202-512-0000 and refer to "USPSTF Expert Panel: Guide to Clinical Preventive Services," Baltimore, Williams & Wilkins, Second Edition.)
Even the most authoritative screening guidelines can be confusing and contradictory. While mammography is widely used to detect breast cancer in women over age 50, for example, there are questions about its effectiveness in women 40 to 49 years old. Organizations including the American Cancer Society, the American College of Radiology, the American College of Obstetrics and Gynecology and the AMA favor screening women in this age group, while organizations like ACP, the American Academy of Family Practice, the USPSTF and its Canadian counterpart, the Canadian Task Force on Periodic Health Examinations, oppose it.
Suzanne W. Fletcher, FACP, professor of ambulatory care and prevention at Harvard Medical School, recommended regular screening of women between the ages of 50 and 70 and discussion of mammography as an option for women aged 40 to 50. In this latter group, the patient's views can play a larger part in the decision.
Dr. Fletcher explained that screening is most effective in women over 50 because 85% of breast cancers occur in that age group. More specifically, it is necessary to screen 5,000 women aged 30 to 34 to diagnose one case of breast cancer, compared with 1,000 women aged 40 to 44, and 500 women aged 50 to 54. She noted that all randomized controlled trials show screening mammography affects survival for women over age 50, while the results for younger women have been contradictory.
When discussing mammography as an option for women between the ages of 40 and 50, Dr. Fletcher said physicians need to realize that younger women tend to exaggerate their risk of getting breast cancer.
When dealing with screening strategies for colorectal cancer (CRC), physicians face less confusion about what age to start screening, but have more choices among screening methods that they must reconcile.
Data assembled since 1992 make it very clear that physicians can prevent a sizable fraction of CRC cases, according to Robert H. Fletcher, FACP, professor of ambulatory care and prevention at Harvard Medical School and co-chair of a government panel reviewing CRC screening guidelines. He explained that most cases of CRC begin as adenomatous, non-malignant colorectal polyps. Their treatment is the key to preventing CRC.
Polyps become more frequent with age, and since about 90% of them occur after age 50, Dr. Fletcher does not regard screening as cost-effective in younger patients. He acknowledged that some individuals are at increased risk of CRC due to family history, adding that more than 75% of CRC cases are sporadic, reducing the need for general screening.
Screening methodology, however, are more confusing:
- FOBT. Dr. Fletcher said that preventive FOBT screening during a 13-year period led to a one-third decrease in deaths from CRC among nearly 50,000 individuals in Minnesota. A report on this controlled study was published in 1993.
FOBT has its downside, however. The procedure requires patients to follow a 48-hour diet that is heavy on fiber and excludes red meat, gastric-irritant drugs, vitamin C, turnips and horseradish. And because slides are rehydrated before testing, the procedure generates a high rate of false-positive results.
- Sigmoidoscopy. A 1992 study found that mortality from CRC was decreased by about two-thirds when the tumor was within reach of the scope, a finding corroborated by two other studies. Dr. Fletcher said that while these trials used a rigid 25-cm scope, the results can be extrapolated to the flexible instruments in use today.
Sigmoidoscopy gives a view of only the distal half of the colon. So, although the combination of sigmoidoscopy and FOBT has not been formally tested, it is safe to assume that combining the two tests is worthwhile, Dr. Fletcher said.
- Barium enema, colonoscopy. Unproved screening tests for CRC include barium enema and colonoscopy. Air-contrast barium enema (ACBE) visualizes all of the colon and is cheaper than colonoscopy. While its value is hard to evaluate, Dr. Fletcher said that it may be only a little less sensitive and specific than sigmoidoscopy. Abnormalities discovered through ACBE need to be followed up with colonoscopy.
Colonoscopy also visualizes all of the colon, but Dr. Fletcher considers it relatively expensive and risky. The procedure is associated with one death per year for every 10,000 persons screened due to perforation and bleeding after biopsy. Trials are now under way to evaluate this procedure.
Dr. Fletcher recommended screening beginning at age 50 with either an annual FOBT, by sigmoidoscopy with a 60-cm flexible scope every three to five years, or by a combination of these two methods. Abnormalities (positive guaiac test or adenomas > 5mm) should be evaluated with colonoscopy.
William A. Check is a freelance writer living in Wilmette, Ill., who writes on medical and scientific topics.
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