Adjusting the obesity thresholds for self-reported BMI in Ireland: a cross-sectional analysis

作者：Madden David^*

来源：BMJ Open, 2013, 3(5): e002865.

DOI：10.1136/bmjopen-2013-002865

Objective: To investigate the optimal adjustment to be made to obesity thresholds when using self-reported body mass index (BMI). %26lt;br%26gt;Design: A cross-sectional study. %26lt;br%26gt;Setting: Data from the Survey of Lifestyle, Attitudes and Nutrition in Ireland, a nationally representative dataset using the Geodirectory (a listing of all residential addresses in Ireland compiled by the postal service) as the sampling frame. %26lt;br%26gt;Participants: A nationally representative sample of 10 364 adults aged 18+, carried out by face-to-face interview with clinical measurement applied to a number of outcomes to a representative subsample of 2174. After discarding the observations with missing values and errors, the eventual sample was 1874. %26lt;br%26gt;Primary and secondary outcome measures: BMI based on measured and self-reported weight and height. %26lt;br%26gt;Background: It is generally found that self-reported BMI understates true or measured BMI and accordingly revised obesity thresholds have been suggested. %26lt;br%26gt;Methods: Data from the 2007 Survey of Lifestyles, Attitudes and Nutrition in Ireland were used to analyse self-reported and measured BMI. The self-reported BMI threshold was adjusted to obtain the optimal signal for measured BMI using different criteria viz. efficiency (maximum number of correct classifications), maximisation of Youden%26apos;s J, maximisation of OR, minimisation of cost of misclassification and constrained optimisation. %26lt;br%26gt;Results: The optimal threshold differed substantially depending on the criterion adopted for choosing it, with thresholds of 29.1 (efficiency criterion), 27.5 (Youden%26apos;s J) and 26.0 (FN rate of 5%). Standard criteria such as Youden%26apos;s J index were shown to implicitly impose relative costs of false-negatives and false-positives which may not always correspond to the values of the analyst. %26lt;br%26gt;Conclusions: When adjusting self-reported BMI thresholds in order to obtain the optimal signal for %26apos;true%26apos; obesity, analysts should explicitly choose the relative costs of false-positives and false-negatives.

• 出版日期2013