How Reliable is the Diagnosis of Obesity?Friday, October 31, 2008
As most readers of these pages probably know, the current definition of obesity is based on the body-mass-index, a number, which, in populations, nicely correlates with body fat.
However, as body fat alone is not the entire picture, other indices that include measurements of fat distribution such as waist circumference or wait-to-hip ration have been suggested, along with cut-offs that would help identify, who is “obese” and who is not.
But how reliably can these indices be measured in clinical practice (as recommended in obesity guidelines)?
This question was addressed by Paul Sebo and colleagues from the Geneva University Hospitals, Switzerland, in a paper just published in Preventive Medicine.
In this study, repeated anthropometric measurements were performed by 12 primary care physicians on 24 adult volunteers, men and women, with an average BMI of 28. While inter-observer reliability for weight, height, and derived BMI were excellent (R>0.99), they were unsatisfactory for waist circumference (R=0.92), hip circumference (R=0.76) and waist-to-hip-ration (R=0.51).
With BMI, only 1% of the volunteers were misclassified as overweight or obese, whereas the use of WC and WHR lead to misclassification in 6% and 23% respectively.
Reliability for the measurements improved after a one-hour training in anthropometric measurements, but the proportion who were misclassified remained high for WC (5%) and WHR (9%).
So, apparently, even with “Swiss precision”, anything that goes beyond height and weight is too complicated to reliably classify obesity in primary care.
But the real question here is whether or not ANY anthropometric measurement can reliably detect who is threatened of affected by excess weight (my definition of obesity). I have argued before that BMI, although fine for population studies, is not useful when making individual decisions about patients.
Not only is there a wide range in individual variability in the actual body fat present in individuals of the same BMI, but, more importantly, there is a huge variability on how that excess fat actually affects that individual’s health.
We have recently proposed the Edmonton Obesity Staging System, which we now use to supplement BMI measurements with stages that reflect the degree of comorbidity and/or reduction in functional status attributable to the excess weight.
Blindly basing decisions to treat or not-to-treat on BMI alone will result in treating a lot of people who have little to gain, while missing out on many who are clearly threatened or affected by excess body fat.