Body Mass Index (BMI) is frequently misinterpreted. Looking around the internet you’ll find plenty of articles and blog posts talking about its failures. Often, the key complaint will be that it can misidentify very muscular individuals as being obese. Additionally, BMI doesn’t take frame size into account, which can be another important factor in assessing weight. The complaints against BMI usually stems from a misunderstanding of the intended use of BMI. The intent is not to attach a specific label to a individual, rather it is used to evaluate populations, and as a screening tool in healthcare settings.
As screening tool, it’s intended to identify individuals who may be in need of further assessment. In the hospital setting, dietitians may use BMI as one criteria for taking time to do a more in-depth review of the patient’s record. The 2008 study Accuracy of Body Mass Index to Diagnose Obesity In the US Adult Population found that, when evaluating obesity, the BMI has 95% specificity for men and 99% specificity for women. This means that if the BMI indicates obesity its highly likely to be correct. This is a great counter-argument against those who bring up false positives like the elite athlete. The sensitivity of the BMI is low, so using it as the sole evidence to rule out obesity would be a problematic.