The interpretation of any clinical laboratory test involves comparing the patient's results with the test's "reference range".

The first step in determining a reference range is to define the population to which the range will apply. A large number of individuals from a group who are thought to represent a "normal" population, will be tested for a particular laboratory test. The reference range is then derived mathematically by taking the average value for the group and allowing for natural variation around that value (plus or minus 2 standard deviations from the average). In this way, ranges quoted by labs will represent the values found in 95% of individuals in the chosen ‘reference’ group. In other words, even in a "normal" population, a test result will lie outside the reference range in 5% of cases (1 in 20). This 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 is not necessarily normal for another group. Indeed for tests such as cholesterol the idea of a normal range has been replaced to a large extent by use of target values, achieved either by lifestyle changes or active treatment.

Whether or not your test result is within the laboratory reference range, the result must be considered within the context of your personal circumstances, and with the benefit of your doctor's knowledge of your past medical history, current medication and the results of any other investigations.