BMI is a population measure that performs poorly at the level of the individual: it does not distinguish muscle from fat and takes no account of fat distribution. Waist circumference and the waist–hip ratio predict cardiometabolic risk better, particularly at a BMI between 25 and 35.
Body mass index
For people of South Asian, Chinese and Japanese origin, lower thresholds apply: overweight from a BMI of 23 and obesity from 27.5, because cardiometabolic risk appears at a lower BMI.
BMI is misleading in athletes with a large muscle mass, in older people with lost muscle mass (where a normal BMI may conceal sarcopenia), in oedema and ascites, and in people under 18 years of age, where iso-BMI or percentile charts should be used.
Waist circumference
Measure the waist between the lowest rib and the iliac crest, at the end of expiration, with the tape horizontal and without compressing the skin.
Ratios and other measurements
| Measurement | Calculation | Threshold |
|---|---|---|
| Waist–hip ratio | Waist / hip | Men > 0.90, women > 0.85 = abdominal obesity |
| Waist–height ratio | Waist / height | > 0.5 = increased risk, irrespective of sex |
| Mid-upper arm circumference | Midpoint of the upper arm | < 23 cm suggests undernutrition in adults |
| Calf circumference | Largest circumference | < 31 cm is a screening marker for sarcopenia |
Body composition
| Measurement | Women | Men |
|---|---|---|
| Body fat, normal | 21–33 % (age dependent) | 8–22 % |
| Body fat, obesity | > 35 % | > 25 % |
| Fat-free mass index | 15–19 kg/m² | 17–22 kg/m² |
Ideal and dosing weight
Drug dosing often requires a weight other than the actual body weight.
- Ideal body weight (Devine): men 50 kg + 0.9 kg per cm above 152 cm; women 45.5 kg + 0.9 kg per cm above 152 cm.
- Adjusted body weight: ideal weight + 0.4 × (actual weight − ideal weight). Used for aminoglycosides and when estimating creatinine clearance in obese patients.
- Lean body weight is used for propofol and rocuronium; actual weight for suxamethonium and for hydrophilic drugs distributed in total body water.
- Body surface area (Mosteller): √(height in cm × weight in kg / 3600) m². Normal adult 1.6–2.0 m².
Nutritional screening
Unintentional weight loss > 5 % over three months or > 10 % over six months, a BMI < 20 in people under 70 years or < 22 above 70 years, and reduced food intake together form the basis of most screening instruments and should prompt a nutritional assessment. See also unintentional weight loss.
