This cross-sectional study is a part of the Musculoskeletal pain in Ullensaker study, where all persons aged 40–79 years in 2010 living in Ullensaker municipality were sent questionnaires regarding musculoskeletal complaints (n = 12.155 of whom 4994 responded (41%)). Height (cm) and weight (kg) were self-reported on the questionnaire with participants being unaware of later measurement. Those who answered "Yes" to the question "Do you have osteoarthritis in the knees, hips and/or hand?" were asked to attend a clinical examination at Diakonhjemmet Hospital, Oslo, Norway. We measured height (cm) and weight (kg) with the participant wearing light indoor clothing, shoes removed and pockets emptied and screened for clinical OA in the knees, hips and hands. A detailed protocol of the study has been published elsewhere. Approval for the study was granted by the Norwegian Regional Committee for Medical and Health Research Ethics (Ref. no. 2008/812a) and the Norwegian Data Inspectorate, and all participants signed informed consent.
BMI was calculated based on both self-reported and measured height and weight (kg/m2). Heights in centimetres and weight in kilograms were both measured once by different project coordinators in a standardized way. We have no data on reliability. When we refer to "self-reported BMI", we mean calculated BMI based on self-reported height and weight. Similarly, when we refer to "measured BMI" we mean calculated BMI based on measured height and weight. Participants were divided into three BMI-categories (measured BMI) according to the World Health Organization. Normal weight was defined as BMI <24.99 kg/m2, overweight as BMI 25–29.99 kg/m2 and obesity as BMI ≥30 kg/m2.
A rheumatologist or medical students screened for clinical OA in the knees, hips and hands according to the American College of Rheumatology (ACR) criteria. Participants were tested only once and no reliability data exist. Those who fulfilled the criteria in either the knees, hips and/or hands were classified as having a clinically meaningful OA diagnosis, whereas those not fulfilling the criteria in any joint were classified as having no clinical OA.
The time interval between the date of self-reporting BMI-data and the date of clinical examination was measured in months. Educational status was used as a measure of socioeconomic status and defined as the highest education level achieved. It was categorized into "primary/upper secondary school" versus "≥1 year at college/university". We used the International Physical Activity Questionnaire score for measuring physical activity level (0–2 scale representing low, moderate and high levels). Smoking status was dichotomized into "never/former smoker" versus "present smoker". The educational status and smoking covariates were dichotomized in order to improve statistical efficiency. This was not done with IPAQ as it is measured on a validated questionnaire with a standardized categorization. Mental health status was measured by the Short-Form (SF)-36 mental summary component score (0–100, higher score = better health).
The differences between self-reported and measured BMI-data were calculated. Close agreement was defined as a difference of +/- 1.00-1.99 kg/m2, whereas exact agreement was defined as a difference of +/- 0.99 kg/m2 or less. A difference of above +/- 2.00 kg/m2 was classified as poor agreement. We examined the percentages agreement between measured and self-reported BMI within each BMI-category and compared participants with and without clinical OA using Chi-square test or Fischer’s exact test. We also examined the absolute differences in BMI, heights and weights across measured BMI-categories for participants with and without clinical OA and compared the groups using independent sample t-tests after having inspected data for normality (examining histograms). A positive difference indicates underreporting (self-reported < measured) and a negative difference indicates overreporting (self-reported > measured).
In multivariate linear regression analyses (robust standard error), we explored whether demographic and clinical factors were associated with the difference between self-reported and measured data for the participants with clinical OA taking the time interval between self-report and measurement into account. We used a descriptive modelling approach not aimed at either explanation or prediction.
P-values ≤0.05 were considered statistically significant. All analyses were performed using STATA IC13.