If a patient's upper arm isn't accessible or you don't have the right size cuff, you may take his blood pressure (BP) from his forearm. But if you assume the reading from the forearm is the equivalent of an upper-arm reading, you may be off base. In research involving 204 medically stable patients, nurse-researchers found that automatic noninvasive BP measurements obtained in the same patient's upper arm and forearm weren't interchangeable. In fact, BP measurements obtained in the forearm differed from those obtained in the upper arm by up to 20 mm Hg.

The researchers acknowledge that forearm measurements may be unavoidable in some circumstances. They advise nurses to clearly document the site used for BP measurements and then use the same site for serial measurements to identify BP trends. They also stress the importance of selecting the proper cuff size to obtain accurate readings.

Source