Depending on the test and factors that may influence its results, reference populations may be chosen based on age, sex, race, general health, and/or medical history.
Next, a large number (minimum of 120) of people who fit the profile of the reference population are tested under nearly identical conditions, and the results are analyzed.
For many tests, reference ranges include the values that are statistically analyzed and reported for the middle 95% of the reference population.To determine ranges, labs may conduct their own studies for the tests they perform, they may adopt reference ranges from test manufacturers or other labs, or they may derive reference ranges from existing patient data.
The most important step in determining a reference range for any test is to define the reference population - the group of people who will be represented in the reference range.
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. 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). The first step in determining a reference range is to define the population to which the range will apply. 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). 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.A large number of individuals from a group who are thought to represent a 'normal' population, will be tested for a particular laboratory test.