Age-related changes in laboratory test results. Clinical implications.

Many quantities assayed in clinical laboratories demonstrate age-related changes. Particularly important periods are early life, adolescence, old age, and after the menopause in females. The changes that occur until adulthood are well documented. Fewer data are available on elderly people even though they consume a large component of healthcare resources. In diagnosis, and prior to initiation of drug therapy, when no previous results are available, reference values must be available to aid interpretation. Reference intervals generated from elderly people are sometimes wider than in younger adults. It is suggested that conventional adult reference values should be used in general for the very elderly since, at least in part, the wider intervals are probably due to inclusion of individuals who are unhealthy in the reference sample group. Most quantities have marked individuality, and serial values for an individual span only a part of the reference interval. Individuals can have values which are very unusual for them but still lie within the reference limits; this implies that clinical laboratory tests will be less than ideal in the detection of latent or early disease. The average within-subject variation in healthy elderly people and younger adults is similar. Therefore, the large database on biological variation can be used, with analytical imprecision, to calculate critical differences for serial results in an elderly individual which must be exceeded before significance can be claimed. These critical differences are of value in monitoring the effects of drug therapy.