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Nuclear Medicine Department, Southern General Hospital, Glasgow G51 4TF, United Kingdom, E-mail: walter.watson{at}sgh.scot.nhs.uk
Dear Sir:
In their recent study, Lee et al (1) developed 2 equations for predicting skeletal muscle mass with use of magnetic resonance imaging in 244 nonobese and 80 obese subjects. In Equation 4 in their paper, the predictive variables were limb circumferences, skinfold thicknesses, height, sex, age, and race. In Equation 6 the predictive variables were body weight, height, sex, age, and race. These relations were first established in a group of nonobese adults (group A), the relations were then cross-validated in a second group of nonobese adults (group B), and, finally, equations developed for groups A and B together were tested in the obese group. Because of the careful cross-validation procedure, the paper is necessarily complicated, but there appear to be some discrepancies in the paper or possibly misinterpretations on my part.
In the Results, Equation 4 is shown to be successfully validated in obese subjects; however, it is stated in the Discussion that there was a significant difference between measured and predicted skeletal muscle in the obese group. In the development of Equation 6, a preliminary equation for group A was found to cross-validate successfully when applied to group B; however, it is stated in the Discussion that "a small bias was observed when the model developed in group A was cross-validated in the nonobese group B subjects."
Perhaps some of the confusion stems from the misapplication of the Bland-Altman (B-A) statistic, which is widely used in method comparisons (2). In this statistic, if Y and X are different estimates of the same entity (normally assessed by using different analytic techniques), then
Unfortunately, Lee et al appear to have correlated the difference between measured and predicted skeletal muscle with the measured skeletal muscle values only (ie, not the average of the measured and predicted skeletal muscle values). Although this seems relatively trivial, it does significantly distort the B-A statistic (3). For instance, if we take Equation 1 above and subtract each side of the equation from X then
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