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Most patients with diseases that can be detected by biochemical profiles present with clinical symptoms and signs of such significance that laboratory tests usually have little screening value. For example, an increase in lactate dehydrogenase (LDH) levels without symptoms is only weakly predictive of underlying disease.

The sensitivity of a test or the rate of detection is probably more sig­nificant. For example, the total serum protein level is not very sensitive for detecting any of the conditions that may alter it; on the other hand, elevated serum cholesterol or blood glucose levels are virtually always detected in a patient with familial hypercholesterolemia or diabetes, re­spectively. Because measuring fasting blood glucose is useful and rela­tively inexpensive, it is a worthwhile screening test for asymptomatic patients with a family history of diabetes or obesity. Also, determining serum cholesterol level is worth the cost, considering the risk for ath­erosclerosis associated with elevated cholesterol and the evidence that lowering the level may reduce cardiovascular morbidity and mortality (see also Ch. 81). The frequency at which these tests should be repeated has not been determined. Some recommend rechecking cholesterol levels in the elderly every 5 yr if the initial cholesterol level is normal, but others advocate remeasuring cholesterol only if the levels are ele­vated or borderline.