Zero end-digit preference in recorded blood pressure and its impact on classification of patients for pharmacologic management in primary care

Abstract

Aim: To document the use of rounding to zero end-digit and assess its potential impact on eligibility for pharmacologic management of CVD risk. Design of study: Cross-sectional study. Setting: A total of 23 676 patients having opportunistic CVD risk assessment in primary care practices in New Zealand. Method: To simulate rounding in practice, for patients with systolic blood pressures recorded without a zero end-digit, a second blood pressure measure was generated by arithmetically rounding to the nearest zero end-digit. A 10-year Framingham CVD risk score was estimated using actual and rounded blood pressures. Eligibility for pharmacologic treatment was then determined using the Joint British Societies' JBS2 and the British Hypertension Society BHS-IV guidelines based on actual and rounded blood pressure values. Results: Zero end-digits were recorded in 64% of systolic and 62% of diastolic blood pressures. When eligibility for drug treatment was based only on a Framingham 10year CVD risk threshold of 20% or more, rounding misclassified one in 41 of all those patients subject to this error. Under the two guidelines which use different combinations of CVD risk and blood pressure thresholds, one in 19 would be misclassified under JBS2 and one in 12 under the BHS-IV guidelines mostly towards increased treatment. Conclusion: Zero end-digit preference significantly increases a patient's likelihood of being classified as eligible for drug treatment. Guidelines that base treatment decisions primarily on absolute CVD risk are less susceptible to these errors.