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Blood Tests and the Impact of Exercise

Exercise can change blood test results.
Exercise can change blood test results.

Before a routine blood test, doctors often ask you to follow a few simple rules. For a fasting blood test, you’re usually asked not to eat 8 to 12 hours before the test. You should also skip drinking juice, coffee and soda. Water is generally good because it keeps more fluid in your veins and makes it easier to draw blood.

Most doctors don’t mention that you might also need to skip intense workouts the day before. High intensity or strenuous workouts one or two days before a blood test can skew blood test results and make it appear like your body is in crisis.

The reference ranges that labs consider normal are based on the general population, not hard-core exercisers. That means even if you’re a moderate exerciser, your results could appear abnormal if you had a particularly hard workout the day before.

The following are various types and intensities of exercises. You and your doctor should consider all these conditions when preparing for and later reading your results.

HIIT and CrossFit

High-Intensity Interval Training (HIIT) or attending a CrossFit class can increase your creatine kinase (CK) levels.

CK is a protein found in every muscle. When muscles are damaged by exercising, CK enters the bloodstream to be broken down by the kidneys. CK levels after a workout can be three to five times higher than usual. Fortunately, those levels drop in half about every 36 hours, so just a couple days’ break can bring the numbers into a normal range.

CK levels ten times normal or higher are referred to as rhabdomyolysis (or rhabdo). This condition is often caused by too much exercise. Damaged muscle tissue releases proteins and electrolytes into the blood. According to the Centers for Disease Control and Prevention:“These substances can damage the heart and kidneys and cause permanent disability or even death. In the workplace, causes of rhabdo include heat exposure, physical exertion or overuse, and direct trauma (e.g., crush injury from a fall).”

The more intense the exercise, the higher those CK levels can go. That’s why doctors need to know how much and how intensely you’ve exercised recently to pinpoint the cause of rhabdo.


Marathoners often push their bodies to extremes. CK levels measured at the end of a marathon have been documented at more than 100 times higher than normal. It could take as long as ten days of recovery for that reading to return to an acceptable level.

Marathon runners also often see increases in albumin, bilirubin, glucose, and protein levels. But not everything causes the numbers to go up. Long runs can DECREASE the inflammation marker c-reactive protein (CRP). While excessive running has been documented to INCREASE CRP and other inflammatory markers.

Some long-distance runners show higher PSA levels when they complete a run, but not all. PSA means prostate-specific antigen, and when it’s elevated, it can indicate prostate disease. So telling your doctor how far you’ve run before a blood test can be essential to rule out a potentially troubling diagnosis. If you’re having a PSA test, it might be best to not exercise for at least 24-48 hours before.

Regular Training

Regular training can also increase your white blood cell count. Our body’s immune system uses white blood cells, so a higher than normal count can indicate you’re fighting off infection. Knowing how frequently you exercise can help the doctor determine if the high numbers point to a problem or are just signs your body is generating more protection against disease.

Weight Training

Weight training can alter the results if you’re taking a thyroid blood test. Elevated thyroid stimulating hormone (TSH) levels can be detected for at least one to three days. Meanwhile, Free T3 (triiodothyronine) levels can drop from that same workout.

Lifting weights can also skew the results of a liver function test. Also known as a liver panel, these tests check the levels of certain enzymes and proteins. Just moderate physical activity can show a noticeable increase in liver function tests. A strenuous workout can alter the results for up to a week.

What To Do

Keep track of all the exercises you do a whole week before your blood test. Then if you go over the results a few weeks later, you’ll have a record of what you did. If you have any concerns, it’s always best to ask your doctor before the test, so you can do what’s necessary to get the most accurate results possible.

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