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How Accurate is the Body Mass Index (BMI)?

This is the time of the year that many employers are sponsoring biometric screening events. This usually encompasses a lab panel, blood pressure reading and measurement of Body Mass Index (BMI). The BMI calculation usually includes height, weight, and waist circumference.

According to Wikipedia, “The body mass index (BMI), is a heuristic proxy for estimating human body fat based on an individual's weight and height. BMI does not actually measure the percentage of body fat. Body mass index is defined as the individual's body mass divided by the square of his or her height.

BMI provides a simple numeric measure of a person's thickness or thinness, allowing health professionals to discuss overweight and underweight problems more objectively with their patients .It is meant to be used as a simple means of classifying sedentary (physically inactive) individuals, or rather, populations, with an average body composition For these individuals, the current value settings are as follows: a BMI of 18.5 to 25 may indicate optimal weight; a BMI lower than 18.5 suggests the person is underweight while a number above 25 may indicate the person is overweight; a person may have a BMI below 18.5 due to disease; a number above 30 suggests the person is obese (over 40, morbidly obese).”

In our practice as Wellness Coaches, we are charged with the responsibility of contacting all of the screening participants to review the results. We usually receive a lot of controversial comments on the BMI value. One of the biggest discrepancies we see when comparing this data from year to year, is that individuals can have wide variances in their height-sometimes by almost a foot. We feel that this is due to the fact that the technician often times has to translate the height into total inches, instead of feet and inches.

It also has been reported to us that the scales used were put down on carpeting or other uneven surfaces. This can cause inaccurate readings, as well. Further, we have heard horror stories of examiners using metal/retractable tape measures to record waist circumference.
Discrepancies in BMI can also occur with extremely muscle bound individuals. All of their other values can be within normal ranges, but the muscles can account for extra weight, much as fat does.

All that being said, an accurate BMI, in conjunction with lab values and blood pressure readings, can be a powerful tool for Wellness Coaches to use as a starting point for lifestyle changes. It is important to recognize and factor in the scenarios mentioned above, when determining the accuracy of the BMI.