It is not unusual for blood pressure to read differently in one arm than another, and according to the Mayo Clinic, a minor difference is considered normal. There are several reasons why blood pressure readings may differ, and most are benign.
Blood pressure is measured in millimeters of mercury (mm Hg). A difference of 5 to 10 mm Hg is considered minor and not cause for concern. A difference more than 10 mm Hg may indicate an underlying problem.
Readings can be slightly higher in the dominant arm. Usually the muscles in that arm are more developed and may produce more resistance against, as well as pressure in, the vessels on that side.
The American Heart Association indicates that a difference in blood pressure more than 10 mm Hg between arms is significantly greater in patients with known coronary artery disease. If you do not have known heart disease, significantly different readings may be the first sign.
A difference of 10 mm Hg or more in blood pressure readings may also indicate a narrowing of the major arteries in the affected arm, per the Mayo Clinic. Consult your doctor if any significant differences in blood pressure are identified.
If blood pressure is higher in one arm, the doctor will take readings from the arm with the higher pressure to ensure an accurate measurement.