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 apparently
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 differences
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.