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Screening tests may pinpoint an increased risk of illnesses such as heart disease or spot early signs of conditions such as cancer. But under the wrong circumstances, they can waste your time and money and lead to unnecessary follow-up tests and treatments. "We have a horrible paradox in this country of underscreening some people for certain diseases, such as colon or cervical cancer, while simultaneously overscreening others," says Michael Hochman of the Keck School of Medicine at the University of Southern California. To help you know when to say yes and when to decline, here's a look at several common screening tests.

EKGs and stress tests: An electrocardiogram, or EKG, records your heart's electrical activity at rest. A stress test does the same as you walk or jog on a treadmill. Abnormal activity may indicate heart disease.

Consider these tests if you have heart-disease symptoms such as chest pain, shortness of breath or a sudden decrease in your ability to perform normal activities (especially if you have a history of heart disease), says Mary Ann Bauman, an internist in Seattle and a spokeswoman for the American Heart Association. You may also need the tests if you have heart-disease risk factors such as diabetes and high LDL ("bad") cholesterol.

Adults at low risk for heart disease can skip these tests. Unclear results may lead to unneeded follow-up treatment such as coronary angiography, which can expose you to as much radiation as 60 to 280 chest X-rays.

Colonoscopy: Here, a doctor checks your rectum and colon for polyps, which are usually harmless. Some of these adenomas can become cancerous, and doctors can remove them during a colonoscopy.

People older than 50 need a baseline colon-cancer screening, says David Lieberman, chief of gastroenterology at Oregon Health & Science University. (People at high risk may need it earlier.) But it doesn't always have to include colonoscopy. Opt for colonoscopy if you've had inflammatory bowel disease for more than eight years; if a parent, child or sibling had colon cancer before age 60; or if you have had a large adenoma removed. (Check with a specialist if you have a genetic condition such as Lynch syndrome.)

For other people, flexible sigmoidoscopy (which looks at your lower colon) or lab tests that check stool for blood may be equally effective, according to a study published in JAMA in 2016.

If your first colonoscopy finds no adenomas or cancer and your risk is normal, get another in 10 years. If one or two adenomas are removed, go in five years. Those older than 75 can skip colonoscopy unless they have a family or personal history of colon cancer or adenomatous polyps.

Imaging for back pain: Anyone with back pain and a history of cancer, unexplained weight loss, chills and fever or sudden loss of bowel or bladder control or leg sensation or strength should have an X-ray, CT scan or MRI of the spine.

Those with lower back pain for less than six weeks can skip it, unless they also have the symptoms mentioned above.

"I still see a lot of patients with two days of back pain and an MRI their primary-care physician ordered," says F. Todd Wetzel, president of the North American Spine Society.

Bone density scan: Women should have a baseline DEXA scan — an X-ray that measures bone density — at age 65, men at 70 if they've lost height, says Jack Ende, president of the American College of Physicians (ACP). Men and women in their 50s and 60s should consider a scan if they have osteoporosis risk factors such as rheumatoid arthritis, smoking or a history of corticosteroid use, Ende says. If results are normal, rescreen in 15 years. For moderate to severe osteopenia (low bone density), go every two years.

Women younger than 65 and men younger than 70 with no risk factors don't require a bone density screen. You don't need to rescreen more than every five years if you're being treated for osteoporosis, according to the ACP.

PSA test: This blood test measures a protein made by the prostate gland and is sometimes used to detect prostate cancer. It's most useful for monitoring men with prostate cancer. And if you have risk factors, such as African American ancestry or a family history of prostate cancer, your doctor may recommend it at age 45.

Many doctors used to encourage it for men at age 50. But the American Urological Association and the American Cancer Society now generally advise that men have it only after talking with a doctor. Draft recommendations from the U.S. Preventive Services Task Force say that men between 55 and 69 should discuss potential harms and benefits of screening with their doctor. A high PSA is often caused by noncancerous conditions, such as an enlarged prostate gland or recent sexual activity, but it may lead to a biopsy anyway. That can cause short-term complications such as bleeding and urinary problems.

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