Association between obesity and high blood pressure: Reporting bias related to gender and age

OBJECTIVE: To examine the validity of self-reported information on obesity and high blood pressure (HBP) in relation to gender and age, and to explore the impacts of their misclassification on the association between obesity and HBP. DESIGN: Community based cross-sectional study. SUBJECTS: 1791 adult subjects living in Humboldt, Saskatchewan, Canada. MEASUREMENTS: Objectively measured HBP was positive if systolic blood pressure (BP) was greater than or equal to 140 mm Hg, diastolic BP was greater than or equal to 90 mm Hg or the subject was currently using antihypertensive medication. Self-reported HBP was positive if the subjects gave an affirmative response to the question:'Has a doctor ever said you had high blood pressure?' Body mass index (BMI) was calculated as weight (kg)/height (m)(2). Obesity was defined as a BMI > 27 kg/m(2). Measured obesity and reported obesity were based on measured and self-reported information on height and weight, respectively. RESULTS: The sensitivity of self-reported HBP was low, and was lower for men than for women, and for younger subjects than for older subjects. The specificity was similar for both genders. Obese individuals had higher sensitivity and lower specificity than non-obese individuals. The differential misclassification of self-reported HBP caused a bias away from the null when the relative risk for HBP in relation to obesity was estimated. CONCLUSIONS: As a result of the gender-and age-related misclassification of self-reported HBP, the modification role of gender and age on the association between obesity and HBP could be altered. The bias caused by self-reported obesity was relatively small and was either toward or away from the null.