Thus, extrapolating from population norms to individuals is less useful for elderly than for younger persons. With age, organ function declines, but the rate of decline varies from person to person and from system to system within one person. Large or sudden changes in function frequently signify a disorder. Determining the likelihood that an abnormal laboratory test result represents a particular disorder may be difficult in the elderly. Pretest probability and a test's operating characteristics can be difficult to determine because precise information about prevalence, sensitivity, specificity, and likelihood ratios for the elderly is often unavailable.
Inaccurate interpretations tend to lead to excess false-positive results.
Drugs commonly used by the elderly may spuriously alter the results of laboratory tests. For example, isoniazid, levodopa, morphine, vitamin C, nalidixic acid, and penicillin G may lead to false-positive urine glucose results. Levodopa may produce a spurious increase in serum bilirubin and uric acid, also leading to false-positive results. A low serum albumin level in healthy elderly persons is usually due to diet. However, low levels may also indicate serious disease, especially when accompanied by undernutrition.