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What is Body Mass Index (BMI)?

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Body mass index (BMI) is used in healthcare as a measurement of body fat and as a health predictor. Calculated by weight/height2, it is easy to find yours on an app or website. We have to include every patients BMI in their charts for insurance billing as well as anesthesia clearance for our surgical patients. But are we all using a flawed calculation?

The concept of a weight/height2 calculation was thought of by statistician Adolphe Quetelet (1796-1874). However the ratio was not used in a nutritional sense until 1972 when Ancel Keys took Quetelet’s Index and coined the phrase Body Mass Index. Keys and his colleagues tested the BMI for accuracy of true body fat and applied it to health outcomes and disease. It slowly gained popularity and in 1985 the National Institute of Health adopted BMI.

Today we use BMI to define our need for health intervention. When your BMI reaches certain levels, you are often considered at increased risk of certain diseases and need more testing.

  • Below 18.5 is underweight.
  • From 18.5 to 24.9 is normal.
  • From 25 to 29.9 is overweight.
  • 30 and over is obese.

Obesity is further classified by the CDC in 3 classes:

  • Class 1: BMI of 30 to < 35
  • Class 2: BMI of 35 to < 40
  • Class 3: BMI of 40 or higher

The Joint Commission sets standards for different procedures that produce predictable outcomes. Hospitals and surgeons must meet these standards in order to get reimbursement. The Joint Commission has set a standard of BMI of <45 for all total knee patients and some hospitals and surgery centers will further limit that to less than 40 per their anesthesia providers. When our patients come into our office, we will discuss their BMI, knee diagnosis, and treatment options.

Is BMI a perfect measuring tool? No, it is flawed for sure. It often does not take into account lean muscle mass that athletes may have, leading to higher BMI. It also does not account for changes from one sex to another, as women tend to have a higher percentage of body fat naturally. It also does not account for children. We are left with an imperfect tool to deal with an imperfect system. However, it is only meant to be an estimate of body fat and health. It is quick, cheap, and readily available to everyone. BMI is still the tool of choice at this time in almost every office, hospital, insurance company, and the CDC. So, for now we will use it for screening, and follow the recommended guidelines as best we can for each individual patient.


1. Commentary: Origins and evolution of body mass index (BMI): continuing saga International Journal of Epidemiology, Volume 43, Issue 3, June 2014, Pages 665–669

2. Adolphe Quetelet and the Evolution of Body Mass Index (BMI) Psychology Today

3 Why BMI Calculation Is Flawed and the History Behind How the Scale Came to Define Obesity