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How To Get the Most Accurate Blood Test Results

male doctor in mask drawing blood from older male adult

A blood test may not be the most pleasant experience in the world, but it is an important part of your overall health plan. There’s a lot your doctor can tell about your health from that little vial of blood. It works somewhat like a crystal ball, but what you do in the hours before could have a negative effect on your blood test results.

The National Institutes of Health remind us that blood tests help doctors check for certain diseases and conditions. They also help check the function of your organs and show how well treatments are working.

Specifically, blood tests can help doctors:

  • Evaluate how well your organs are working
  • Diagnose diseases and conditions such as cancer, HIV/AIDS, diabetes, anemia and coronary heart disease
  • Find out whether you have risk factors for heart disease
  • Check whether medicines you're taking are working
  • Assess how well your blood is clotting

Blood tests are very common and have minimal risk. But there are some things that you can do just before the test that can affect the outcome, says Robert Krajcik, MD, a family medicine specialist with UH Ohio Medical Group.

Here are some of the things Dr. Krajcik recommends that you should do, or avoid, to get the most accurate blood test results.

For Accurate Blood Test Results: Fast

Most people are familiar with having to fast before a blood test. There is good reason your doctor recommends not eating or drinking: to attain the most accurate blood test results.

Nutrients and ingredients in the food and beverages you eat and drink are absorbed into your bloodstream. This could impact factors measured by certain tests. Fasting improves the accuracy of those tests, Dr. Krajcik says.

Blood tests that you will likely need to fast for include:

  • Blood glucose test
  • Cholesterol test (total, HDL, LDL)
  • Triglyceride level test

“Fasting ensures your blood test results will not be influenced by foods you eat in the hours before your blood is drawn,” Dr. Krajcik says. “The results could be affected because the food may not be fully absorbed by your body. You must give it eight to 12 hours to digest.”

Water Does Not Impact Blood Test Results

Some tests will require you to eat nothing at all, but in most cases, you will be allowed to drink water. Water can actually help with the blood test Dr. Krajcik says.

“Blood is roughly 50 percent water, and the water you drink hydrates your veins,” he says. Water plumps the veins up, making it easier for the technician to find them, and, as a result, making for a smoother blood draw. Water will also help keep your blood pressure from dropping.

Ask your doctor what you’re allowed to drink before your test.

Don’t Exercise For Accurate Blood Test Results

This may be the only time in your life your doctor may tell you not to exercise. A workout can negatively impact blood test results. For the most accurate blood test results, your blood should be drawn when you’re rested, Dr. Krajcik says.

A workout before a fasting blood test can alter the results of cholesterol and glucose tests. “In some cases of cholesterol testing, if you exercise before having blood drawn, your LDL cholesterol levels may actually increase,” he says. “This is the bad cholesterol that you want to keep low.”

Although exercise improves an overall blood panel, strenuous physical exercise before a blood draw may negatively impact the results, so hold off on that workout until you’re done.

Alcohol Could Affect Blood Test Results

In general, alcohol the night before should not affect your blood test results, Dr. Krajcik says. If you begin fasting 12 hours before the test (including alcohol), it will most likely be metabolized, as long as you keep it to a drink or two.

However, if the panel is specific to your liver enzymes, they may be altered. Your best bet is to ask your doctor, or when in doubt, just leave it out.

Medications and Blood Test Results

The first rule of medications and blood test results is to talk to your doctor about what medicines you’re taking before your blood draw. You should also mention it to the person who is drawing your blood, so they can make a note of it, too.

You should never stop taking a prescription unless your doctor tells you.

Some medicines can affect blood test results, but this doesn't necessarily mean you should stop taking your medicine. Steroids, for example, can increase your cholesterol levels, but your doctor can take this into account when reading your results.

Vitamins, supplements, and even herbal remedies can also affect results, so if you’re taking any of these you should tell your doctor.

Smoking Affects Blood Test Results

To get the most accurate blood test results you should not smoke.

There have yet to be any strong clinical studies on the effect of e-cigarettes on blood sugar levels of people with diabetes, but research shows higher nicotine levels are associated with slightly increased HbA1c levels in people without diabetes.

In general, if you’ve been asked to fast before your blood test, you should avoid smoking, too.

Routine Blood Tests Are a Good Idea

Even if you feel healthy, it’s still a good idea to get a regular blood test. Your blood test results can tell you a lot about your health, Dr. Krajcik says. The results will alert you to potential problems, or may spur you to consider some healthy lifestyle changes.

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