The authors conducted this study to: (1) determine the effect of age on orthostatic vital signs; and (2) to define the sensitivity and specificity of alternative definitions of "abnormal" orthostatic vital signs in blood donors sustaining an acute 450-mL blood loss. The population studied were 100 healthy adult volunteer blood donors and 100 self-sufficient ambulatory citizens attending a senior citizens daytime activity center. Subjects with a history of orthostatic hypotension were excluded. Subjects were first placed in the recumbent position and their rate pulse and blood pressure were determined after 1 minute; these same parameters were measured in the same arm beginning 30 seconds after standing. In blood donors measurement of orthostatic vital signs was repeated immediately after blood donation. Blood donors served as their own controls in the determination of sensitivities and specificities. Mean orthostatic vital sign changes were as follows: pulse rate, 2 +/- 7 beats per minute; systolic blood pressure, -3 +/- 9 mm Hg; and diastolic blood pressure, 1 +/- 7 mm Hg. There was no clinically meaningful variance in orthostatic blood pressure changes with age. For a given specificity, pulse rate increase was the most sensitive of the orthostatic vital signs used alone; a pulse rise of greater than 20 beats per minute had a sensitivity of 9% with a specificity of 98%.(ABSTRACT TRUNCATED AT 250 WORDS)