Approach

BMI measurement

The most common method to diagnose obesity is the body mass index (BMI), defined as weight divided by height squared ([weight in kg]/[height in m]²).[1][2][3][39] The BMI is a reasonable determination of obesity in the vast majority of people, but it is not accurate in pregnancy or in people with a large amount of muscle mass (e.g., professional athletes). In addition, the BMI does not account for sex, age, or bone structure, all of which can influence the relative amount of body fat. Despite these drawbacks; however, the BMI remains the standard and usual test for the diagnosis of obesity in adults. 

Waist circumference

Elevated waist circumference (>102 cm in men and >88 cm in women) is commonly used as an indicator of risk for obesity-related disease and appears to be associated with an increased risk of hypertension, diabetes, dyslipidaemia, and the metabolic syndrome.[1][8][9][10]

Laboratory evaluation and imaging

No laboratory evaluation or imaging study is necessary for the diagnosis of obesity per se. If rare, secondary causes of obesity or conditions associated with obesity are suspected, then laboratory testing, including full blood count, serum electrolytes, serum transaminases, thyroid function tests, and liver function tests, may be performed. An ECG may be done if there are clinical signs suggestive of heart disease, and an abdominal ultrasound scan performed if fatty liver is suspected. Polysomnography may be ordered if obstructive sleep apnoea is suspected.

Body composition tests

While some experimental methods can determine body composition, none have become simple, inexpensive, and popular enough to be put into general use. These tests include skinfold measurement, hydrodensitometry, bioelectric impedance analysis, dual-energy x-ray absorptiometry, and others.[93]

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