Search

Background: Rounding blood pressure (BP) to the nearest 10 mmHg (terminal digit preference) and selecting for particular values near treatment cut‐offs (number preference) have both been previously described. Both reduce measurement accuracy, and may have consequences for treatment and survival. Aim: To check for number preference in screening for hypertension, and whether this influenced subsequent mortality. Design: Prospective case‐control screening study. Methods: In the General Practice Hypertensive Study Group (GPHSG), prospective case control study patients ( n =23 574) were screened on one occasion for high phase‐IV diastolic BP (DBP4) (≥90 mmHg). Identified cases were matched with normotensive controls for age, sex, date of screen and ethnic group, and were registered for mortality follow‐up ( n =6310). Patients with a high DBP4 had two further readings, and were treated if it remained elevated. Results: For DBP4 terminal digit, ‘0’ was over‐represented (28.2% vs. 20%), and the number ‘88’ was over‐represented in both men and women. There was an excess adjusted death rate for females with DBP4 88–89 mmHg vs. 90–99 mmHg for both cardiovascular (RR 2.56, 95%CI 1.43–4.56, p =0.0015) and all‐cause (1.56, 95%CI 1.06–2.29, p =0.023) mortality. For males, the corresponding rates were non‐significantly reduced: cardiovascular RR 0.69, 95%CI 0.42–1.14, p =0.15; all‐cause RR 0.93, 95%CI 0.68–1.27, p =0.64. Discussion: The quality of BP measurements should be monitored both in research studies and in clinical practice as part of clinical governance procedures.

Terminal digit preference is a well-known source of error in blood pressure measurements. This was demonstrated in general practice computer databases by conducting a routine search and constructing a frequency distribution of the 20 992 blood pressure readings using mathematical software for a personal computer. The charts produced have a practical application in providing an audit tool whereby nurses and doctors can be made aware of this source of error, providing an opportunity for improving measurement technique and developing more refined and realistic protocols for blood pressure management in general practice. The same principles may be applied to other measurements made in the GP consultation such as peak expiratory flow rate.