TERMINAL DIGIT PREFERENCE, RANDOM ERROR, AND BIAS IN ROUTINE CLINICALMEASUREMENT OF BLOOD-PRESSURE

We examined the presence, magnitude, and consequences of systematic an d random errors caused by terminal digit preference in the measurement of highest systolic blood pressure during prenatal visits in 28,841 n on-referred pregnant women who delivered between 1 January 1982 and 31 March 1990. In the overall distribution of terminal digit readings, 7 8% were read to 0, 15% to even digits other than 0, 5% to 5, and only 2% to odd digits other than 5. This preference for 0's was consistent across the entire distribution of blood pressure and for a variety of maternal characteristics. The relative frequency of the cutoff value o f 140 mmHg (i.e. the percentage of readings on 140 mmHg) within the ra nge containing the value (i.e. 138-142 mmHg) was similar to the relati ve frequency of other multiples of 0. This was true whether the compar ison was made in the overall study sample, or in a pre-selected low-ri sk subgroup or high-risk subgroup, indicating no systematic bias. On t he other hand, a strong tendency to read blood pressure values to the nearest 0 had a marked effect on the classification of hypertension. C hanging the definition of hypertension from greater-than-or-equal-to 1 40 mmHg to > 140 mmHg produced a reduction in prevalence of hypertensi on from 25.9 to 13.3% in the overall study sample, from 15.4 to 6.3% i n the low-risk subgroup, and from 43.3 to 25.3% in the high-fisk subgr oup. Epidemiologic studies that compare prevalences of hypertension in different populations based on routine clinical measurement of blood pressure and a single cutoff point should assess the consequences of t erminal digit preference in defining hypertension. The relative freque ncy and changing cutoff point approach might be helpful in evaluating the existence and consequences of systematic bias and random error cau sed by terminal digit preference in other routine clinical and laborat ory measurements as well.