Adjustment of blood pressure data by season.

Seasonal variation in blood pressure was examined in 801 males of age 50 years and over. Systolic and diastolic pressures revealed significant seasonal patterns with peaks in spring and late autumn and troughs in mid-winter and summer. The blood pressure data standardized for age and body weight (kg/m2) and adjusted for irregular fluctuations showed that the range of variation in systolic pressure within a year was 8-9 mmHg and that in diastolic pressure 5 mmHg. Careful analysis of the data indicates that the variations in blood pressure attributable to the month of the year are at least as significant as those due to age and body weight. An example is drawn from an actual study which demonstrates how estimates of the prevalence of hypertension may be substantially biased if the seasonal pattern in blood pressure is not taken into account. It is therefore suggested that this factor should be allowed for routinely, at least in local epidemiological surveys, in addition it should be recognised as important in daily clinical work--especially in those patients who are in need of antihypertensive treatment.