health-1“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,” says geriatrician Suzanne Salamon, M.D., assistant professor at Harvard Medical School.

About Normal Ranges And Interpreting The Numbers

When you look at a printout of your lab results, you’ll find the normal ranges for each blood test next to your personal 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 a 9.1 mg/dL, right in the middle, you can feel confident that your calcium level is normal.

But 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,” warns Salamon. “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 created as your body breaks down the protein in your diet. Excess urea is removed from the blood by the kidneys, so high levels of BUN in the blood can indicate that kidney function is declining. So, what does it mean 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,” says 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 reflect: recent infections, medication side effects, stress, gender, age and 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 actually be a sign of a bleeding ulcer in the stomach or small intestine, not failing kidneys.

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

When To Be Concerned

Instead of looking at a one-time test result on the high or low end of normal, Salamon says 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,” Salamon says.

For example, let’s say you have your blood sugar (glucose) measured every three years (that’s how often the American Diabetes Association recommends routine blood sugar testing for everyone over age 45, if previous test results have been normal). The normal range is between 60 and 100 mg/dL. Levels between 100 and 125 mg/dL are considered “prediabetes” – that is, not quite full-blown diabetes, but approaching it. If your results have usually been 81 or 82 mg/dL, but your latest result is suddenly 98 mg/dL, it’s still considered normal, but the change might spark a conversation about your diet. Salamon says she would, for example, ask you questions like “Have you put on weight? Are you eating more carbohydrates and sugary foods?” as a way to start investigating why your blood sugar is suddenly going up.

But some tests don’t have any room for fluctuation, such as tests for liver enzymes, called AST (aspartate aminotransferase) and ALT (alanine aminotransferase).

“If these levels were always normal, and now one of them starts to go up one or two points, there might be something wrong with liver function, and I’d want to know why,” says Salamon.

What You Should Do

Get an annual check-up, and don’t skip 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. If you have some results at the high or low end of normal, ask your doctor if you should repeat the test or investigate them further. But remember: “Each test has its own rules, and it takes years to understand how the numbers work together,” says Salamon. “Leave the interpretation up to your doctor.”

Heidi Godman is the executive editor of the Harvard Health Letter.