Both types of monitors are validated by the British Hypertension Society and the Association for the Advancement of Medical Instrumentation. Patients wear the monitor for a 24-hour period, usually a workday. The monitor is preprogrammed to record BP, usually every 15 to 20minutes during daytime hours and every 20 to 30minutes during night-time hours. Patients are instructed to keep an activity log throughout the testing period for evaluation of stress- and activity-related BP changes. Pickering et al.409 ABPM: Currently available ambulatory monitors are fully automatic and can record BP for 24 hours or longer while patients go about their normal daily activities. Most monitors use the oscillometric technique. They can be worn on a belt or in a pouch and are connected to a sphygmomanometer cuff on the upper arm by a plastic tube. Subjects are asked to keep their arm still while the cuff is inflating and to avoid excessive physical exertion during monitoring. The monitors are programmed to take a reading every 15 to 30 minutes throughout the day and night. At the end of the recording period, the readings are downloaded into a computer. Standard protocols are used to evaluate the accuracy of the monitors, and approved devices are usually accurate to within 5 mm Hg of readings taken with a mercury sphygmomanometer. The daytime level of ambulatory BP that is usually considered the upper limit of the normal range is 135/85 mm Hg. The Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure143 ABPM is warranted for evaluation of white-coat hypertension in the absence of target-organ injury. It is also helpful in patients with apparent drug resistance, hypotensive symptoms with antihypertensive medications, episodic hypertension, or autonomic dysfunction. Ambulatory BP values are usually lower than office readings. 