Estimating BMI for Patients with Amputations

A depiction of body segment proportions and revised standards, along with a formula to estimate BMI for patients with an amputation was published in the February 1995 Journal of the American Dietetic Association. These tools, based on research from the late 1800’s and re-evaluated in 1955 and 1969, are among one of the methods still in use today for assessing BMI in individuals who are missing part or all of a limb. Available studies suggest that body components for men and women are essentially equivalent; however, adjustments for age or ethnicity have not been evaluated.

This formula for estimating BMI for patients with an amputation is:

WtE = Wto/(1-P)

WtE represents the estimated total body weight, Wto the observed or current body weight and P is the proportion of total body weight represented by the missing body segment

Example:

Patient is an 85- kg person with one leg amputated at the knee. According to the chart, P = (1-.059) or 0.941. 85kg/0.941 = 90.3kg WtE.

The estimated weight may then be used to determine BMI using tables, a nomogram or BMI calculation.

Alternative methods have also been proposed to assess BMI for individuals with amputations, such as upper arm anthropometry, corrected arm muscle area (CAMA), and mid upper arm circumference (MUAC). However, studies are small, and applications limited to those with lower limb loss.

Dual-energy X-ray absorptiometry (DXA) scans are often considered a gold standard in determining lower body fatness in adults with lower limb loss, but access to these tools may not be feasible in many settings. One tool that has been used by the Department of Defense (both with individuals with limb loss and those without) is the circumference method. The circumference method, based on research from the 1970s and 1980s, has not been validated for those with limb loss. However, its equations have been validated against DXA and other fat mass measurements for accuracy in individuals without limb loss. The advantage of the circumference method is that it only requires two to three circumference measurements on an individual rather than relying on special equipment. These measurements are around the neck and abdomen, and for women they also include the hips. The mean measurements for each site are then entered into an equation to determine percent body fat.

A study published online March 17, 2021 in the Journal of the Academy of Nutrition and Dietetics compared the circumference method and DXA in individuals with and without lower limb loss. Although the DXA measurements were significantly higher in both cases, further analysis found that the two tools had “good to near excellent agreement” with each other for estimating percent body fat among men in this population. As with other assessment methods, inconsistency in measurement can occur limiting its usefulness, so a practitioner's clinical judgment is warranted.

The authors suggested that the circumference method may be a useful tool to estimate percent body fat when other methods, such as DXA, are not available. It could also serve as a way to track changes in body composition; however, this is an area for ongoing research and the aforementioned study was limited to males in the military and was not representative of all types of amputations.

References:

• Current Perspective on Assessment of Human Body Proportions of Relevance to Amputees. Journal of the American Dietetic Association. Vol. 95. No. 2. Page 215.
• Academy of Nutrition and Dietetics. Nutrition Care Manual. Musculoskeletal, Amputations. Accessed May 20, 2021.
• Frost AP, Norman Giest T, Ruta AA, Snow TK, Millard-Stafford M. Limitations of body mass index for counseling individuals with unilateral lower extremity amputation. Prosthetics and Orthotics International. 2017;41(2):186-193.
• Miller M, Wong WK, Wu J, Cavenett S, Daniels L, Crotty M. Upper-arm anthropometry: an alternative indicator of nutritional health to body mass index in unilateral lower-extremity amputees? Arch Phys Med Rehabil. 2008 Oct;89(10):2031-3.
• George BG, Pruziner AL, Andrews AM. Circumference Method Estimates Percent Body Fat in Male US Service Members with Lower Limb Loss. J Acad Nutr Diet. 2021 Mar 17:S2212-2672(21)00101-5.