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To examine trends in sensitivity and specificity of self-reported vs clinically measured body mass index (BMI) from three adult national lifestyle surveys over a 10 year period.
Design and Participants
Cross-sectional studies involving nationally representative samples in 1998, 2002 and 2007. Data on both self-reported and measured height and weight were available from 66 men and 142 women in 1998, 147 men and 184 women in 2002 and 909 men and 1128 women in 2007.
Interview based survey/measurement in mobile clinic or household of three representative samples in Ireland.
Main Outcome Measures
Trends in sensitivity and specificity in BMI classification of normal weight, overweight and obesity based on self-reported vs clinically measured height and weight.
Sensitivity scores for the normal category improved across time but decreased for the overweight (75.3%→67.6%→66%) and obese categories (79.5%→64%→53.4%), demonstrating a decrease in the capacity for self-diagnosis of obesity. Simultaneously, specificity levels in the obese category remained consistently high (100%→98%→98.2%). In all three surveys, the sensitivity for the normal BMI category was high (>94%) indicating truly healthy subjects self-reported as healthy. Sensitivity and specificity varied by gender. Trends in measured obesity levels increased between SLÁN1998 and SLÁN2002 (21.2%–26%) but decreased to 24.4% in SLÁN2007. The differential between self-reported and measured obesity increased across the three surveys (4.4%→7.9%→10%).
While the capacity for self-recognition of normal BMI has improved over time, increasing numbers of overweight and obese people underreport their weight. Problems of misclassification are greatest in the overweight and obese categories resulting in an underreporting of true overweight and obesity levels in the Irish population. The declining sensitivity, accompanied by rising levels of obesity in the population, suggests social norms may be at play, and has important implications for future public health interventions.