The benefits and risks of various screening tests have long been debated in the medical community. Now, with evidence that full-body CT scans expose individuals to dangerously high doses of cancer-causing radiation, the debate is going public.

To really understand the debate, the term "screening test" must be distinguished from the term "diagnostic test." Although most of the testing methods are the same, the reasons for testing differ in important ways.

Diagnostic tests look for disease in people who have signs or symptoms of a health problem or disease. Examples of this:

• A throat swab to check for strep throat in someone with a sore throat and fever.

• A CBC (complete blood count) for someone who is very fatigued, looking for signs of anemia, infection or other causes.

• A biopsy of a suspicious-looking mole to check for skin cancer.

• A CT (computed tomography) scan of the abdomen to check for appendicitis or another cause of abdominal pain and tenderness.

Screening tests look for early signs of disease in seemingly healthy people who have no symptoms. They are done because the disease is serious and common enough in the population that testing susceptible people has been shown to be beneficial and often saves lives because early treatment can be given. Examples of widely accepted screening tests include:

• Mammograms to look for early signs of breast cancer in women.

• The PSA (prostate-specific antigen) blood test and digital rectal exams, which can help detect prostate cancer in men.

• Pap smears, done for early diagnosis of cervical cancer.

• Sigmoidoscopy, colonoscopy and fecal occult blood tests to look for signs of colon cancer.

• Blood tests for cholesterol and blood pressure checks help determine risk of heart disease.

The full-body CT scan currently in the news has been around for a few years.

These are offered at radiology clinics, which have sprung up across the United States. No referral from your health care provider is needed. Health insurance rarely covers the cost of these scans, so individuals pay cash - often $1,000 or more.

What kind of people are willing to pay this amount and subject themselves to a full-body CT scan, even though they are healthy? No statistics have been kept, but it's believed that hundreds of thousands of Americans have had this test, looking for cancer and heart disease.

People who undergo unnecessary screening tests on their own dime usually fall into the category of "worried well." These are people who are so concerned about having a disease that they often see many different doctors and have dozens of medical tests in an attempt to get a diagnosis.

The worried well may obsessively read medical books and articles and scan the Internet for information about various diseases. They insist on the latest tests, and can't take "no" for an answer if their health care provider declines to order a test. They simply move on to a new health care provider who will do what they ask.

These full-body scans and other screening tests are a dream come true for the worried well, and for the owners and radiologists who profit from the tests.

Occasionally an unnecessary test will reveal a disease that can be treated and save a life, but this happens so rarely that the risks and costs outweigh the possible benefits.

Any screening test will have a certain rate of false positive results, so some healthy people who have a full-body scan will be told they may have a disease.

They will have to undergo more extensive, and often expensive testing - usually paid for by health insurance companies - only to find that nothing can be found.

Kathryn B. Brown is a family nurse practitioner with a master's degree in nursing from OHSU. Is there a health topic you would like to read about? Send ideas to kbbrown@eastoregonian.com. You can find more local health news and information in the Health section at www.dailyastorian.info.

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