Wrist blood pressure monitors can be accurate if they are used correctly and are properly calibrated.
Some people with very large arms may not have access to a well-fitting arm cuff at home. If so, measuring blood pressure at the wrist may be better. Wrist blood pressure monitors may also be an option for people who had lymph nodes removed from the armpit (axiliary lymph node resection).
However, there are some factors that can affect the accuracy of the readings, such as the position of the arm and wrist, the size of the cuff, and the type of device used.
Using a wrist blood pressure monitor at home often gives falsely high readings due to poor positioning. If you use one, place it directly over the wrist (radial) artery, where you can feel the pulse. Don't place it over clothes. Keep your wrist at heart level. Be still during the test and don't bend the wrist. Bending (flexing) the wrist can cause incorrect readings. It is also important to ensure that the cuff is properly fitted, as a cuff that is too loose or too tight can also affect the accuracy of the reading.
The following operation instruction diagram may help you better in the way of how to use the wrist blood pressure monitor correctly: