Health care professionals use a variety of tools to determine a person's risk of certain diseases and conditions. One of these tools is the body mass index, or BMI. The BMI is a primary screening tool because it is inexpensive, states the Centers for Disease Control and Prevention.
Function
A person's BMI is a comparison of his height and weight. Medical professionals use the BMI to determine if a patient is at a healthy weight. Doctors often use the BMI in conjunction with other tests, such as blood glucose tests, complete blood counts, medical history and other diagnostic tests to determine if a patient is at risk for obesity-related conditions.
Considerations
The BMI method isn't an appropriate tool for everyone. Children, body builders and athletes, and the elderly may have to use alternatives to the BMI, according to Medline Plus. Children have special growth patterns that the traditional BMI chart doesn't factor in. Body builders generally have a lower body fat percentage for their weight than an inactive person of the same weight. The elderly may need a little more weight than a younger person. Body fat measurements, waist circumference and hydrostatic weighing are some of the alternatives.
Formula
Many health care professionals use a chart to quickly determine a patient's BMI. For those who don't have a BMI chart, it is possible to determine the BMI by hand. Multiply your height in inches by your height in inches. Divide your weight by the result of the previous step, then multiply that result by 703. For a person who is 63 inches and weighs 125 pounds, calculate: 63 times 63 equals 3969; divide 125 by 3969 to get 0.0315; multiply that by 703 to get a BMI of 22.14.
Categories
There are five categories for the BMI. A person is considered underweight if the BMI is under 18.5. When the BMI is between 18.5 and 24.9, the person is at a healthy weight. An overweight person is someone with a BMI of 25.0 to 29.9. Someone with a BMI of 30.0 to 39.9 is considered obese. Anyone with a BMI of 40.0 or higher is considered morbidly obese.
Warning
The risk of obesity-related health conditions increases as the BMI increases. Varicose veins, heart disease and type 2 diabetes are some diseases associated with obesity. The risk of having a stroke and developing sleep apnea also increases as the BMI increases. Elevated blood pressure and unstable blood sugar levels are also possible. These risks can be lowered by losing weight--even as little as 10 percent of the current weight, according to the American Cancer Society.
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