For many tests, there is no single reference range that applies to everyone because the tests performed may be affected by the age and gender of the patient, as well as many other considerations. For example alkaline phosphatase is an enzyme found in the cells that make bone, so its concentration in the body rises in proportion to new bone cell production. In a child or adolescent, a high alkaline phosphatase is not only normal but also desirable because the child should be growing healthy bones. But these same concentrations 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 patient’s gender is another important consideration for some tests for example muscles contain an enzyme called creatine kinase. Because of their larger muscle mass, men tend to have higher CK concentrations in their bloodstream than women.