Values recorded indirectly (auscultatory method) correlate quite well with simultaneous direct intraarterial
recordings (r = 0.94 to 0.98). Still, Korotkoff phase I sounds do not appear until 4–15 mmHg
below direct systolic blood pressure, whereas Korotkoff phase V sounds disappear above the direct
diastolic value (by 3–6 mmHg). Hence, there is some minor underestimation and overestimation.
Physicians may also cause inaccuracies. For example, despite previously agreeing to use three readings for
diagnosis, a group of British general practitioners diagnosed hypertension after only one measurement
in half of the cases.
Similarly, 37% of German ambulatory physicians determined diastolic pressure using Korotkoff phase IV (muffling), rather than the more accurate phase V. Still, the most common physician’s error is failure to use sufficiently large cuffs. In one survey, only 25% of primary care offices had them available. Of interest, auscultatory automatic monitors have fewer discrepancies than experienced clinicians.
Finally, in some patients the blood pressure measured in the physician’s office is considerably and
consistently higher than the daytime ambulatory value. This phenomenon is called the “white coat” effect and is seen in as many as 10–40% of untreated and borderline hypertensive patients. Even treated patients often show blood pressure differences that are >20/10 mmHg.
The phenomenon is more pronounced in female than male patients and results more often in responses to the white coat of doctors than to that of nurses.