For many tests there is no single reference range that applies to everyone. Some tests may be affected by the age and sex of the person as well as many other considerations.

An example is alkaline phosphatase, which is an enzyme found in the cells that make bone, its levels change with bone growth. In a child or adolescent, a high alkaline phosphatase level is not only normal but also desirable because the child should be growing strong, healthy bones. The same levels found in an adult are a sign of trouble. So, experience from testing large numbers of people has led to different reference ranges for different age groups.

The person’s sex is another important consideration for some tests, for example muscles contain an enzyme called creatine kinase. Men have more muscle and so they tend to have higher creatine kinase levels in their blood than do women.