Selective recording in blood pressure readings may increase subsequent mortality

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) (> or =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.