Reference Values for Blood Pressure of Healthy Schoolchildren in Shiraz (Southern Iran) using Quantile Regression

Background: The Reference values of systolic and diastolic Blood Pressure (BP) levels of school children aged 6–11 years by two different analytic strategies are presented and compared. Methods: From the cross-sectional study a total of 2064 children (52.3% boys and 47.7% girls) aged 6–11 years living in Shiraz (southern Iran) and considering their sex and height were used for this analysis. Polynomial Regression (PR) and Quantile Regression (QR) models based on Restricted Cubic Spline (RCS) were performed to calculate age and height specific reference ranges. To assess comparability of the two techniques, a chi-square goodness-of-fit within sex and age groups was preformed for each method. Results: Both statistical methods generated reference values of systolic and diastolic BP using data from ap- parently healthy children. Analysis of data by two approaches reflected an increase in BP measurements with age and height in both sexes based on a nonlinear manner up to age 11. We found 50th and 95th percentile dif- ferences by two methods in BP level between the tallest and the shortest individuals, ranging from 2-7 mmHg. Conclusion: Using the QR model based on RCS offered the most flexible and better fit than PR model. The advantages of the QR led to a better adaptation of reference limits to the original data. This statistical approach might be preferable for the calculation of reference ranges in particular by non-normal distributed variables. Our results might help clinicians reach a consensus on the definition of hypertension in Iranian children living in Shiraz, south of Iran.