ARCHIVED CONTENT: As a service to our readers, Harvard Health Publishing provides access to our library of archived content. Please note the date each article was posted or last reviewed. No content on this site, regardless of date, should ever be used as a substitute for direct medical advice from your doctor or other qualified clinician.
Picture this: you're reading the results of your recent bloodwork, and you notice some numbers are teetering on the edge of the normal range. Should you be concerned? "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 Dr. Suzanne Salamon, 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 Dr. 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," says Dr. Salamon.
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 Dr. 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
- inaccurate lab procedures.
Dr. 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 about blood test results
Instead of looking at a one-time test result on the high or low end of normal, Dr. 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."
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. Dr. 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.
What you should do
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," says Dr. Salamon. "Leave the interpretation up to your doctor."