The United States Preventive Task Force reviewed 27 studies in February of 2014 to determine the benefits and harms of screening for high blood pressure and concluded that office blood pressure readings are not accurate about half of the time. This leads to over-diagnosis and over-treatment of blood pressure. For this reason they recommended ambulatory blood pressure (ABPM) readings, which involve wearing a blood pressure monitor for 24 hours and obtaining about 80 readings throughout the day and night.
In my practice I often will ask people with high office readings to take their BP at home twice a day for about 2 weeks and to then email the readings to me for a more accurate assessment of their blood pressure. In some people this creates more anxiety, so I then recommend ABPM testing.