Problems in interpreting laboratory tests. What do unexpected results mean?
It is always important that physicians not overreact to apparently abnormal laboratory values by undertaking inappropriate further investigations or clinical treatments. When confronted with unexpected differing test results from repeat testing in the same individual, physicians should be aware of explanations other than laboratory error and change in the patient's clinical status. While test-related variables may be factors, intraindividual biologic variation is much more common and may be the explanation for discrepant results. For this reason, physicians need to know which laboratory tests are associated with significant intraindividual biologic variation as well as the magnitude of possible changes. Age-associated physiologic changes may significantly alter certain laboratory values in the elderly without constituting a pathologic process. Laboratory values that may appear abnormal in 10% or more of the healthy elderly without necessarily representing a pathologic process include serum alkaline phosphatase, fasting blood glucose, 2-hour postprandial glucose, erythrocyte sedimentation rate, hemoglobin, and a normal serum creatinine level in the face of a markedly decreased creatinine clearance. To ensure proper assessment of the geriatic patient, the clinician needs to be aware of these age-related changes and possible effects on laboratory values. More clinical research is needed to establish appropriate reference ranges, especially for those over the age of 75 years.