"Your test was out of the normal range," your doctor says to you, handing you a sheet of paper with a set of test results, numbers on a page. Your heart starts to race in fear that you are really sick. But what does this statement mean, "Out of the normal range"? Is it cause for concern? The brief answer is that a result out of the normal range is a signal that further investigation is needed.

Tests results—all medical data—can only be understood once all the pieces are together. Take one of the simplest medical indicators of all—your heart rate. You can take your resting heart rate right now by putting your fingers on your pulse and counting for a minute. Most people know that the "average" heart rate is about 70 beats per minute. How do you know what a "normal" heart rate is? We know this on the basis of taking the pulse rate of millions of people over time.

You probably also know that if you are a regular runner or are otherwise in good physical condition, your pulse rate could be considerably lower—so a pulse rate of 55 could also be "normal". Say you walk up a hill—your heart rate is now 120 beats a minute. That would be high for a resting heart rate but "normal" for the rate during this kind of activity.

Your heart rate, like any medical observation, must be considered in context. Without the proper context, any observation or test result is meaningless. To understand what is normal for you, your doctor must know what is the normal heart rate of most other people of your age, and what activity you are doing at the time—or just before—your heart rate is measured.

The interpretation of any clinical laboratory test involves an important concept in comparing the patient’s results to the test’s "reference range." (It’s also commonly called the "normal range" but today reference range is considered a more descriptive term, for reasons explained on the next page.)