Agreement between body mass index, waist circumference and skin-fold thickness in the United Kingdom Army

Background: Body Mass Index (BMI) does not distinguish between fat-free mass and fat mass, yet this distinction is important as a clinical tool, especially in the military and occupations for which physical fitness is important. Aim: This study assessed the level of agreement between BMI, sum of skin-folds and waist circumference in UK Army personnel. Subjects and methods: Three hundred and eighty-six males were selected to study the level of agreement between BMI, waist circumference, waist circumference/height and sum of skin-folds at four sites. As the units of measurement differ, this study generated standardized distributions with skewness of zero. Results: The limits of agreement between BMI and waist circumference were fairly wide (between boundaries 1.75 z-score) and wide between BMI and sum of skin-folds (2.57 z-score) and waist circumference and skin-folds (2.37 z-score). The standard deviations of the differences of BMI and waist circumference decreased with increasing means of these two measures. The kappa statistic for BMI and waist circumference was 0.7 and similar to 0.5 if compared against sum of skin-folds. Conclusion: The level of agreement between BMI and waist circumference is acceptable regardless of method to assess obesity, but would be poor for assessing thinness. The level of agreement with sum of skin-folds was mediocre.