In some patients it is difficult to accurately assess blood pressure because of a "white-coat effect" that results in an elevated measurement when blood pressure is tested by medical personnel. In these cases, self-monitoring by patients or family members is recommended to obtain more representative blood pressure measurements. However, a few patients report even higher blood pressures when self-monitoring than when they are monitored by health professionals. This "inverse white-coat" phenomenon could be an intrinsic response to the act of measuring blood pressure or a result of poor measurement technique.
—Anne D. Walling, "Accuracy of blood pressure self-monitoring," American Family Physician, November 1, 2003

Racing pulses at the doctor's office are so common, there's even a name for it — "white coat hypertension." Unfortunately, that spike in blood pressure can make it difficult for doctors to tell how patients are really doing.

A study in the New England Journal of Medicine this month suggests a portable device that tracks blood pressure day and night to get a more realistic reading for hypertension patients. The multiple readings could let doctors decide more accurately who needs aggressive treatment to prevent complications and who can be spared medication.

Up to one-third of patients diagnosed with high blood pressure show the "white coat" effect during exam-room readings.
—"Pulse points," The Boston Globe, June 24, 2003

When all else fails, make sure the person really has high blood pressure. The doctor must bear in mind that the mere act of visiting a physician can elevate one's blood pressure, especially if the person had been rushing around, had to wait an hour reading five-year-old National Geographics because the doctor was running late or, commonly, just because many people are nervous when doctors get within a few feet of them. The phenomenon is known as the "white coat" effect.
—Dr. Howard Seiden, "Many factors can affect high blood pressure reading," The Toronto Star, May 14, 1987