Introduction: In clinical practice, end-digit preference is a common feature of blood pressure (BP) measurements. A wider use of electronic BP measuring machines could decrease this observer-linked artefact. The purpose of this analysis was to investigate the frequency of end-digit preference and to evaluate the impact of this observer bias on the assessment of the BP control induced in a large group of hypertensive patients treated with a calcium-channel blocker in whom BP was measured either with an automatic device or with a conventional sphygmomanometer.
Methods: Five hundred and four physicians participated in the study and 2199 patients were included. Treatment with lercanidipine was introduced at a dosage of 10 mg and titration to 20 mg was optional according to the physician's decision. BP was assessed at 4 and 8 weeks. To measure BP, physicians could use either a standard mercury sphygmomanometer or a pre-defined validated semi-automatic device (Microlife Average Mode, BP 3AC1-1, Microlife Corporation, Berneck, Switzerland) but they had to use the same method throughout the study. Physicians had to transcribe all BP measurements onto case report forms.
Results: Very marked digit preferences were observed for both the conventional and the automatic measurements, being most prominent for the digit "0" (52% and 25%, respectively) followed by a preference for the digit "5" (19% and 15%). The use of the semi-automatic device reduces to a certain extent the frequency of the bias but the problem remains if physicians have to transfer the BP values onto case report forms. The end-digit preference has a major impact on the evaluation of a treatment effect and on the assessment of the percentage of patients achieving target BP in a population.
Conclusion: These results confirm that end-digit preference remains a serious bias in clinical practice. This bias has important consequences when evaluating the efficacy of a new antihypertensive drug. There is a need for training programmes and quality controls in clinical practice. The development of automatic systems with a direct transfer of BP values from the measuring device to the clinical chart or to the case report form should be encouraged.