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

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 level is not only normal but desirable—the child should be growing healthy bones. But these same levels found in an adult are a sign of trouble—osteoporosis, metastatic bone disease (extra bone growth associated with tumors), or other conditions. It is because of these significant variations due to age that the few reference ranges that you may see on this site do not include ranges for children or adolescents. Experience from testing large numbers of people has led to different reference ranges by age group.

Hemoglobin and hematocrit (a red blood cell measure) both decline as a natural part of the aging process.

The patient’s sex is another important consideration for many tests:

• Creatinine is produced as a natural by-product of muscle activity and then removed from your bloodstream by your kidneys. It is often measured as a gauge of how well your kidneys are functioning. Because males have greater muscle mass than females, the reference range for males is higher than for females.
• The form of the enzyme creatine kinase called CK-MB presents a similar situation. It is released into the bloodstream by damaged muscles, and a high level of CK-MB indicates damage to the heart muscle, so this enzyme is one of the indicators used to diagnose heart attacks. Because of their greater muscle mass, men tend to have higher CK levels. When the test first came into use, many elderly women demonstrated considerably lower levels of CK-MB and, thus, did not pass the threshold level believed to indicate a heart attack, so heart attacks were often missed in this age group of women.

As another example, blood loss through menstruation may cause lower hemoglobin and hematocrit levels in premenopausal women. These are examples of tests with reference ranges keyed to both age and sex.