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The accuracy of many medical tests can depend on factors such as the time of day, your posture during the test, whether you’ve been sick, your medications and whether you recently rode a bicycle or had sex. And inaccurate results can lead to more than unnecessary medical bills. When tests fail to detect a health problem, they can deprive you of needed care. When they trigger false alarms, they can lead you to take drugs or even undergo procedures you may not need.

You might assume that your doctors and nurses would tell you what you should and shouldn’t do before a test, but that doesn’t always happen. Here’s how to make sure you get the best results from two key tests:

Cholesterol: Fast first?

The results of your cholesterol test can vary by as much as 10 percent, depending on what you recently ate or drank, the drugs you take and other health problems you may have. For example, some research suggests that periods of high stress can raise your cholesterol levels. Here’s how to do the test right:

Ask whether you need to fast. Your doctor might just want to check your total cholesterol and HDL (good) cholesterol levels, which aren’t affected by eating. But if your doctor also wants to look at your LDL (bad) cholesterol levels and triglycerides (a type of fatty acid found in the blood), you’ll need to fast for about 12 hours beforehand, which means no food and no drinks other than water, says Nieca Goldberg, a cardiologist and director of the Joan H. Tisch Center for Women’s Health at the NYU Langone Medical Center.

Redo if results are abnormal. If your cholesterol levels are unusually high or low, your doctor should repeat the test a few weeks later. “Stress can raise cholesterol, but other conditions, including cancer, can dramatically lower it,” Goldberg says. If it’s still high, ask your doctor about lifestyle measures or medication to reduce it. If it’s low, your doctor may want to run tests to rule out other health problems.

Avoid intense workouts. Some research suggests that exercising strenuously 12 to 24 hours before the test could falsely elevate your HDL.

Be savvy about home tests. Testing cholesterol levels at home might be useful, especially if you are on a cholesterol-lowering drug. But you shouldn’t need to test more frequently than every six months, and if you do test at home, look for a test that breaks down cholesterol into LDL, HDL and triglycerides.

Make sure the doctor knows all of your meds. Drugs such as birth control pills, hormone replacement therapy, steroids and blood pressure medications such as beta-blockers and diuretics can raise cholesterol levels.

Blood pressure: Your posture matters

Sometimes your blood pressure might be high at the doctor’s office but not when you monitor yourself at home. That could be due to “white-coat hypertension,” a spike in blood pressure that can occur if you get nervous at your doctor’s office — although it could also signal a risk of developing high blood pressure long term. Here’s how to do the test right, whether at home or in the doctor’s office:

Double-check your drugs. Certain medications, such as over-the-counter decongestants, prescription amphetamines and the steroid prednisone, can increase blood pressure. So make sure your physician knows all of the drugs you take.

Go to the bathroom first. A full bladder can raise blood pressure by as much as 15 points systolic (the top number) and 10 points diastolic (the bottom number).

Sit correctly. Your back should be straight and supported, with you seated on a chair rather than, say, on the examining table. Your feet should be flat on the floor, with your legs uncrossed. Your arm should be supported on a flat surface (such as a table) with the upper arm at heart level.

Measure twice. If your blood pressure reading is high, your doctor or nurse might measure your pressure again at the end of the exam, when you’re usually more relaxed, Goldberg says. The lower reading is usually more accurate.

Remain quiet. Talking can raise your blood pressure by up to 10 points on either the top or bottom number.

Check the cuff. Measurements taken over clothing or with too small a cuff can boost blood pressure by as much as 50 points.

Copyright 2016. Consumers Union of United States Inc.

For further guidance, go to www.ConsumerReports.org/Health, where more detailed information, including CR’s ratings of prescription drugs, treatments, hospitals and healthy-living products, is available to subscribers.