Doctors are fond of referring to doctor's-office measurements of blood pressure as "the gold standard" for making decisions about the diagnosis of hypertension and the success of treatments.
In fact, measurements made in a clinical setting are probably the most misleading blood pressure data taken.
Here are the guidelines (as stated by the National Institutes of Health , the American Heart Association , and other organizations) for taking proper blood pressure measurements:
1) The patient should be seated and should rest and relax for five full minutes before the measurement is taken.
2) The patient should sit with feet planted on the floor.
3) The patient's arm should be supported at heart level.
4) The blood-pressure cuff should have a bladder large enough to encirle 80 percent of the arm.
5) The patient should not be engaged in conversation during the measurement (and probably not during the five-minute rest period, either).
It's not uncommon for measurements taken by a nurse or doctor to violate all five of these guidelines at once. In fact, three research projects showed that no physicians studied followed all of the American Heart Association guidelines for measuring blood pressure, and few of them were followed at all . Think about it:
1) Five minutes rest before the measurement is almost never given. Five minutes rest doesn't mean sitting in the waiting room and then being summoned to an immediate blood pressure test. It means sitting and resting in the chair where the blood pressure will be taken. This can make a huge difference, and, for most people, the sooner their blood pressure is taken after sitting down, the higher their blood pressure will be. It's common for the readings to drop by 5 points with a few miutes of rest; for people who are apprehensive or have "white-coat syndrome," some slow breathing and relaxation can drop the readings by 10 points or even more.
2) Hop up on the exam table! What does it matter if your feet dangle?
3) And, again...hop up on the exam table! Your arm doesn't really need to be supported, does it? Well, according to Pickering, et al , "The position of the arm can have a major influence when the blood pressure is measured; if the upper arm is below the level of the right atrium (when the arm is hanging down while in the sitting position), the readings will be too high. Similarly, if the arm is above the heart level, the readings will be too low. These differences can be attributed to the effects of hydrostatic pressure and may be 10 mm Hg or more, or 2 mm Hg for every inch above or below the heart level." In other words, your arm position can make all the difference in whether or not your doctor decides you have hypertension and starts you on a course of medications.
4) The "standard" cuff is good up to an upper-arm diameter of 13 inches. That might seem like plenty, but it isn't; arm diameters higher than 13 inches are common in people who are overweight, and also very common in men who are carrying even a modest amount of muscle. (I am not overweight, nor am I very tall, nor am I built like a bodybuilder, but my upper arm diameter is more than 14 inches.) How much difference does cuff size make? In one study , using a cuff one size too small or large resulted in 30-40% of patients being misdiagnosed as hypertensive or normotensive.
A comprehensive discussion of cuff-size problems has been assembled by Kmom, whose website (though oriented toward the blood-pressure problems of pregnant women) provides a thorough overview, references, and plenty of anecdotes.
5) No talking? Ha. I have never, ever, no, not even once, had my blood pressure taken without the doctor or nurse holding a conversation with me. And why should it matter? According to research, talking raises blood pressure about 7 points above merely sitting and relaxing. Seven points might not seem like a big deal, but on average blood pressure medications only lower blood pressure by about 10 points. In other words, shutting up might have an effect nearly as large as taking a powerful medication.
Notice that most of these mistakes--taking blood pressure without a rest period, wrong leg and arm positions, using a cuff too small, and talking during measurements--tend to raise blood pressure. This means that for most people the blood pressure readings taken in a doctor's office are on the high side.
This has only been discovered since the advent of non-invasive ambulatory blood pressure monitoring (ABPM). ABPM systems are worn continuously (usually for 24 hours), and take blood pressure readings without an inflating cuff; the patient is unaware when blood pressure measurements are being taken. Data from ABPM have opened a whole new window on the topic of blood pressure. Blood pressure throughout the day varies considerably more than most people ever expected (and most doctors seem to be unaware of this fact).
One of the important discoveries is that--as might be expected from the foregoing--the blood pressures taken in doctor's offices tend to be higher than average blood pressures. Most important has been the revelation of the extent of "white-coat hypertension;" on the order of 20% of patients diagnosed with high blood pressure have perfectly normal blood-pressure readings outside the doctor's office. And, although many doctors and nurses continue to believe that white-coat hypertension affects only the systolic blood pressure, both systolic and diastolic pressures can be affected: one study found rises of 9 to 23 mm systolic, and 3 to 10 mm diastolic . (I'll have more to say about white-coat hypertension in a future post.)
In short, measurements made in your doctor's office are likely to be too high. If you really want to know what your blood pressure is, you'll need to measure it at home. Surprisingly, the American Heart Association, which long contended that the only reliable means of blood-pressure evaluation was clinical measurement, now recommends home monitoring as a critical corrective to the over-high measurements taken in doctor's offices.
In other words, even the doctors are beginning to realize their readings aren't accurate.
Don't think for a minute, though, that knowing this will prevent them from prescribing your medication on the basis of their office readings.
 National Institutes of Health, Blood Pressure Evaluation and Treatment, NIH Publication 03-5233. December 2003
 Pickering, Thomas G, et al, Recommendations for Blood Pressure Measurement in Humans and Experimental Animals Part I, Hypertension. 2005; 45:142
 Campbell, Norman R.C., and McKay, Donald W., Accurate Blood Pressure Measurement: Why Does it Matter?, CMAJ. August 1999; 161 (3) 277
 Sprafka, JM et al. The Effect of Cuff Size on Blood Pressure Measurements in Adults. Epidemiology. May 1991; 2(3):214-7.
 Pickering, Thomas G., White Coat Hypertension: Time for Action. Circulation. 1998; 98:1834-1836.
 Pray, W. Steven, Home Blood Pressure Monitoring, US Pharmacist. 2008; 33(2):14-17.