K nowing which health-related screening tests you'll benefit from—and when it's best to have them—can pose a significant challenge. Many are important, pinpointing an increased risk of illnesses such as heart disease or spotting 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, M.D., of the Keck School of Medicine at the University of Southern California.

Case in point: Almost half of women in their 50s at low risk for osteoporosis have been screened for this bone-thinning disease. But only 58 percent of women in the U.S. between 65 and 74 (who should undergo this screening) have been, according to a study published in 2015 in the Journal of General Internal Medicine.

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

The test: An electrocardiogram (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.

Who should have it: 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 your normal activities (especially if you have a history of heart disease), says Mary Ann Bauman, M.D., a cardiologist 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.

Who can skip it: Adults at low risk for heart disease. The U.S. Preventive Services Task Force (USPSTF), an independent panel of national experts who offer evidence-based guidance about preventive health practices, recommends against routine EKGs and stress 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

The test: A doctor checks your rectum and colon for polyps (masses of tissue) with a flexible tube. Polyps are usually harmless, but some, known as adenomas, can become cancerous. Doctors can remove them during a colonoscopy.

Who should have it: Those over age 50 need some kind of baseline colon-cancer screening, says David Lieberman, M.D., chief of gastroenterology at Oregon Health & Science University. (People at high risk may need it earlier.)

But that baseline screening doesn't always have to include a colonoscopy. Opt for one if you’ve had inflammatory bowel disease for more than eight years; a parent, child, or sibling with colon cancer before age 60; or a large adenoma removed. Check with a specialist if you have a genetic condition like Lynch syndrome.

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

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.

Who can skip it: Those over age 75, unless they have a family or personal history of colon cancer or adenomatous polyps.

Imaging for Back Pain

The test: An X-ray, CT scan, or MRI to take pictures of your spine.

Who should have it: 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.

Who can skip it: Those with lower back pain for less than six weeks, unless you're in the group above. Usually, back pain eases with walking, ibuprofen (Advil and generic) or naproxen (Aleve and generic), heat, and, if pain lasts more than a few weeks, physical therapy.

“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, M.D., president of the North American Spine Society.

Bone Density Scan

The test: A DEXA scan is an X-ray that measures how dense your bones are.

Who should have it: Women should have a baseline scan at age 65, men at 70 if they’ve lost height, says Jack Ende, M.D., 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.

Who can skip it: Women under age 65 and men under age 70 with no risk factors. You don’t need to rescreen more than every five years if you’re being treated for osteoporosis, according to the ACP.

PSA Test

The test: This blood test measures prostate-specific antigen (PSA), a protein made by the prostate gland, and is sometimes used to detect prostate cancer.

Who should have it: It’s most useful for monitoring men with prostate cancer. If you have risk factors, such as a family history of the disease or you’re African-American, your doctor may recommend it at age 45.

Who can skip it: Many doctors used to encourage it for men age 50 and older. But the American Urological Association and the American Cancer Society (ACS) now generally recommend that they consider it only after talking with a doctor. While the USPSTF is currently updating its guidelines, the panel's draft recommendation says men between 55 and 69 should discuss potential harm and benefits of screening with their doctor, and decide whether a PSA is right for them.

A new analysis of two older studies, published in the Annals of Internal Medicine, suggests that one of those studies underestimated the PSA's ability to save lives. But the numbers are modest and the potential downsides of the screening remain. (See Consumer Reports' explanation of that research here.)

For instance, a high PSA is often due to 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, says Richard Wender, M.D., of the ACS.

What About Pap Smears and Mammograms?

The timing of these screening tests can be confusing and may vary depending on your history.

Pap smears: Many women in the U.S. get this test annually, which examines cervical cells for abnormalities that may lead to cervical cancer. But it’s unnecessary to have this screening test every year, according to the American College of Obstetricians and Gynecologists (ACOG) and ACS.

If you’re between age 21 and 29, have a Pap test every three years­. Between age 30 and 65, ACOG recommends having a Pap test every three years or a Pap test and a human papillomavirus test every five years. You can stop after age 65 if you’ve had three normal Pap tests in a row or you no longer have a cervix.

Mammograms: Experts generally agree that women should begin having these breast X-rays­—which may identify breast cancer­­—no later than age 50. Talk with your doctor about whether to be screened annually or every other year.

If you have risk factors (obesity, smoking, and especially a family history of the disease), it’s reasonable to start screening in your 40s, says Marvin M. Lipman, M.D., Consumer Reports’ chief medical adviser. If you’re 75 or older, ask your doctor whether continued testing makes sense for you. The American Cancer Society recommends continuing if you have a life expectancy of 10 years or more.

3 Routine Tests to Question

These screening tests are often given to healthy people but haven’t shown benefits in the absence of specific health concerns.

Manual pelvic exams: Your doctor feels your uterus and ovaries, searching for signs of abnormalities. But a review of studies published in JAMA Internal Medicine in 2014 found there was no evidence that these exams pick up conditions such as ovarian cancer earlier, and they make women feel physically uncomfortable and anxious.

Routine chemistry screens: Doctors often order tests to check blood levels of things like electrolytes, proteins, and kidney and liver function as part of a routine physical. But they can generate false alarms resulting in, for example, a biopsy you might not need.

Routine urinalysis: Unless you have signs of a urinary tract infection, such as frequent, burning urination, there’s no need to do this during a routine physical. You may end up being incorrectly diagnosed with a UTI and be treated with antibiotics even though you don’t have an infection.

Editor's Note: This article also appeared in the October 2017 issue of Consumer Reports on Health.