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As a smart medical consumer, you know it’s important to look over the results of your routine blood work, even when the numbers are all within the normal range.

But what if you notice that some results are at the high or low end of that range? Should you be concerned about this?

“It’s tricky, because in some tests, a borderline result makes no difference. In others, it might indicate an important change in health that we need to follow or act on,” said geriatrician Suzanne Salamon, M.D., assistant professor at Harvard Medical School.

Each lab establishes its own normal ranges for blood test results. The ranges are based on various factors, such as the makeup of the local population where blood is drawn, the instruments used to look at the specimens, and the technologies used to separate various components in your blood.

When you look at a printout of your lab results, you’ll find the normal ranges for each blood marker next to your personal blood test results. For example, if your routine blood work includes a test for calcium in the blood, your lab may list the normal range for calcium as 8.3 to 9.9 milligrams per deciliter (mg/dL). If your result is 9.1 mg/dL, right in the middle, you can feel confident that your calcium level is normal.

Interpreting the numbers

What if a blood test result is at the very low or high end of normal, or even slightly outside the normal range? Is that a red flag?

“Don’t jump to conclusions,” Salamon said. “Blood test results can vary a little bit, depending on the lab. And many people are consistently on one side or the other of the normal range, and for them, that’s healthy.”

Take, for example, a routine measure of blood urea nitrogen (BUN), a waste product from the breakdown of protein you eat. Excess urea is removed from the blood by the kidneys. High BUN levels can indicate that kidney function is declining.

So what if your BUN level is at the very high end of the normal range? “If I see that it’s borderline high, I might ignore it,” said Salamon. “It’s common for BUN to go up if you don’t drink enough, and that can happen when someone is fasting before having blood drawn.”

Minor fluctuations in test results may also result from recent infections, medication side effects, stress, gender, age or inaccurate lab procedures.

Salamon emphasizes the need to look at someone’s entire picture of health to interpret a blood test. In our BUN example, a number just above the normal range might be a sign of a bleeding ulcer in the stomach or small intestine, rather than failing kidneys.

“It depends on the person, the symptoms, and the other conditions that are present,” she said.

When to be concerned

Instead of looking at a one-time test result in the high or low end of normal, Salamon said she looks at trends.

“I get concerned if there’s a change from what’s been normal for years, for you. If your test result is always in the high normal range, I’m not concerned. But if it’s always been in the low normal range, and today it’s high normal, that’s different.”

Get an annual check-up, and don’t skip the routine blood work. Even if you feel that you’re healthy, it’s still a good idea to have a continuous record of standard blood markers, so your doctor can look for trends.

What happens if you have some worrisome results at the high end of normal? “I might repeat the test,” said Salamon. “If it is still a concern, we can investigate the cause.”

And the great news is that staying on top of certain numbers on the edge of normal can help you keep from developing chronic disease.

For example, if your blood sugar numbers are rising within the normal range, you have plenty of time to start exercising and losing weight to bring them back down. And even if they’re in the prediabetes range of 100 to 125 mg/dL, you can still make lifestyle changes to keep from progressing to full-blown diabetes.

— Harvard Health Letter