Is the blood pressure the same in both arms?

It is generally believed that there are blood pressure differences between the right and left arms, but most of the validation studies have used nonsimultaneous measurements. It is important to know whether there is a difference, as methodological comparisons are often made using a different technique on each arm. We have measured the blood pressure simultaneously in both arms in 91 patients, using two observers and two cuffs to provide eight measurements in a fully balanced design. The mean difference in blood pressure between the arms (left/right) was -1.3 (SE 0.4)/1.1 (SE 0.3) mmHg. Mean differences of the same magnitude were found between the two observers, and between the cuffs. Thus, no systematic difference of any consequence was found, although individual patients had mean between-arm differences in blood pressure of up to 10 mmHg. These results suggest that no significant bias is introduced in methodological comparisons that use both arms.