The interpretation of any clinical laboratory (or pathology) test involves comparing the person's results with the "reference range", or "normal" range for that test.

The first step in determining a reference range is to define the population that the test result range applies to. A large number of individuals individuals who are thought to be the "normal" population are then tested. The reference range for a particular laboratory test is then worked out mathematically by using the average value for the “normal” polulation group together with the variation around that value (plus or minus 2 standard deviations from the average). In this way, ranges quoted by laboratories represent the values found in 95% of individuals in the chosen “reference” group. This means that even in a "normal" population a test result is outside the reference range in 5% of cases (1 in 20). It is why the term "reference range" is preferred over "normal range".

When you examine test results from different populations, you quickly discover that what is "normal" for one group of people is not necessarily normal for another population group. For tests such as cholesterol the “normal range” has been replaced to a large extent by use of target values, achieved either by diet and lifestyle changes or active treatment.

Whether or not your test result is within the laboratory reference range, consider it within the context of your personal circumstances and with the benefit of your own and your doctor's knowledge of your past medical and personal history, together with the results of any other investigations performed.