When to worry about blood test results

Abnormal readings aren’t always a problem

Published: May 2014

Not long ago I received an urgent phone call from a patient, a 67-year-old investment broker, anxiously asking, “What’s with this high potassium value? Should I make out my will?”

Under a new federal directive, commercial labs are required to give patients copies of their laboratory data on request, and the broker had asked for his. He received the report so promptly that I hadn’t even had a chance to look at my copy.

I brought his chart up on the computer screen, and sure enough, there it was: a serum potassium level slightly above the “reference,” or normal, range—with a menacing “H” (for “high”) next to it. I knew there was no medical reason for that result. I asked him whether the tech­nician had a difficult time drawing blood. He replied that ­indeed she had, leaving the tourniquet in place for quite a while. I reassured him and sent him to get another blood specimen drawn without a tourniquet.

Why? Because I suspected that the tourniquet had been left in place so long that it caused the red blood cells, which contain lots of potassium, to leak the mineral into his serum, the clear-liquid part of the blood. The second test, without a tourniquet, produced a normal result.

What’s ‘abnormal’ anyway?

Blood tests are an indispensable tool of medicine. They’re used to screen, diagnose, confirm, and monitor diseases. There are many hundreds of them, and each one is subject not only to observation but also to interpretation.

There is no such thing as a perfect lab test, but all tests done today use methods that have been scrutinized and approved by the Food and Drug Administration. And the federal government certifies and inspects the labs regularly to make sure they’re keeping up standards.

Those “reference ranges” you see on the reports may vary slightly from lab to lab, depending on which of several possible methods a lab chooses to use. Test results can also fall outside the reference range for reasons that have nothing to do with disease or medications.

One common example: ­Patients who zealously follow the usual instruction to have “nothing to eat or drink the night before” can end up with high blood urea nitrogen (BUN), which assesses kidney function. Their kidneys are fine, but they’ve dehydrated themselves enough to cause an elevated BUN reading. That’s why I tell my patients to have “nothing to eat or drink from dinner the night before—except for liberal amounts of water.” As a bonus, adequate hydration makes it easier to draw blood by plumping up the veins.

And triglycerides and blood glucose levels, included in many routine blood tests, are very sensitive to the time of the last meal and reach the “fasting” baseline only after 8 to 12 hours without eating. So patients who had that late-night snack and have their blood drawn early the next morning might have out-of-range ­results.

Other abnormal readings may or may not indicate a problem. A high lev­el of creatine phosphokinase, a muscle enzyme, can signal a heart or muscular ailment—or it can mean you worked out shortly before your blood was drawn.

And if you’re an adolescent or recovering from a broken bone, a high level of alkaline phosphatase, a bone (and liver) enzyme, is completely expected; but if you’re not, it could be an ominous sign of liver disease or certain cancers.

So even if you exercise your option to receive your results directly from the lab, you should still get your doctor to weigh in and not try to decode the data yourself.

Take charge of your tests

Before that next needle stick, ask your doctor:

• Why am I having this test?

• How reliable is it in diagnosing or ruling out the disease?

• What happens next if the test is positive? Negative?

• Are there specific instructions about food, drink, medication, or exercise in advance of the test?

• Is the lab a federally certified facility?

• How soon will the test results be available, and how do I get them?

Marvin Lipman, M.D.

Chief Medical Adviser and Medical Editor
Editor's Note:

This article also appeared in the June 2014 issue of Consumer Reports on Health.

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