Some tests provide a simple yes or no answer. Was the culture positive for strep throat? Did the test find antibodies to a virus that indicates an infection?

But for many more tests, the meaning of the results depends on their context. A typical lab report will provide your results followed by a reference range. For example, your results for a thyroid-stimulating hormone (TSH) test might look something like: 2.0 mIU/L, ref range 0.5 – 5.0 mIU/L. The test results indicate that it falls within the "normal" range.

How was that reference range established? The short answer is: by testing a large number of healthy people and observing what appears to be "normal" for them.

The first step in determining a given reference range is to define the population to which the reference range will apply, for example, healthy females between 20 and 30 years old. A large number of individuals from this category would be tested for a specific laboratory test. The results would be averaged and a range (plus or minus 2 standard deviations of the average) of normal values would be established.

The term "reference range" is preferred over "normal range" because the reference population can be clearly defined. Rather than implying that the test results are being compared with some ill-defined concept of "normal," the reference range means the results are being considered in the most relevant context. When you examine test results from different populations, you quickly discover that what is "normal" for one group is not necessarily normal for another group. For example, pregnancy changes many aspects of the body’s chemistry, so pregnant women have their own set of reference ranges.