Both are accurate. They are accurate under different conditions.
Upper arm
The clinical reference. The brachial artery is large and close to the surface, so readings are less sensitive to position. Choose this for daily readings at home.
Wrist
Compact and portable, but the wrist must be held exactly at heart level. Choose this for travel, or if an arm cuff is uncomfortable or does not fit.
The short answer
If you can use an arm cuff, use an arm cuff. If you can’t, a wrist monitor used correctly is a good second choice.
Not a diagnosis
Home readings are for monitoring. Discuss unusual results with your physician and never change medication on your own.
