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Reference Ranges and What They Mean

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Effects of Age and Sex

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.

Some examples of variation in reference ranges based on age:

  • 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.

Examples of reference range variation based on sex:

  • Creatinine is produced as a natural by-product of muscle activity and is removed from your bloodstream by your kidneys. Creatinine levels will be affected by a person's muscle mass as well as their kidney function. 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 enzyme creatine kinase (CK) and one of its forms called CK-MB present a similar situation. CK is released into the bloodstream by damaged muscles; CK-MB is released into the bloodstream when the heart muscle is damaged. Therefore, 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 and the level of CK-MB that indicates a heart attack in men is higher than for women. When the test first came into use, the reference range was based on the higher levels. Many elderly women being tested for a heart attack demonstrated considerably lower levels of CK-MB (because of their smaller muscle mass) and, thus, did not pass the threshold level believed to indicate a heart attack; so heart attacks were often missed in these women.

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