Psychological Medicine

Original Articles

Eighty-five per cent of what? Discrepancies in the weight cut-off for anorexia nervosa substantially affect the prevalence of underweight

J. J. Thomasa1a2a3 c1, C. A. Robertoa1 and K. D. Brownella1

a1 Department of Psychology, Yale University, New Haven, CT, USA

a2 Klarman Eating Disorders Center, McLean Hospital, Harvard Medical School, MA, USA

a3 Eating Disorders Clinical and Research Program, Massachusetts General Hospital, Boston, MA, USA


Background DSM-IV cites <85% of expected body weight (EBW) as a guideline for the diagnosis of anorexia nervosa (AN) but does not require a specific method for calculating EBW. The purpose of the present study was to determine the degree to which weight cut-off calculations vary across studies, and to evaluate whether differential cut-offs lead to discrepancies in the prevalence of individuals who are eligible for the AN diagnosis.

Method Two coders independently recorded the EBW calculation methods from 99 studies that either (a) compared individuals with AN to those with subclinical eating disorders or (b) conducted AN treatment trials. Each weight cut-off was applied to a nationally representative (n=12001) and treatment-seeking (n=189) sample to determine the impact of EBW calculation on the proportion who met the AN weight criterion.

Results Coders identified 10 different EBW methods, each of which produced different weight cut-offs for the diagnosis of AN. Although only 0.23% of the national sample met the lowest cut-off, this number increased 43-fold to 10.10% under the highest cut-off. Similarly, only 48.1% of treatment seekers met the lowest cut-off, whereas 89.4% met the highest.

Conclusions There is considerable variance across studies in the determination of the AN weight cut-off. Discrepancies substantially affect the proportion of individuals who are eligible for diagnosis, treatment and insurance reimbursement. However, differences may not be fully appreciated because the ubiquitous citation of the 85% criterion creates a sense of false consensus.

(Received October 02 2007)

(Revised June 22 2008)

(Accepted July 12 2008)

(Online publication September 08 2008)


c1 Address for correspondence: J. J. Thomas, Ph.D., Eating Disorders Clinical and Research Program, Massachusetts General Hospital, WAC-816D, 15 Parkman St, Boston, MA 02114, USA. (Email:


This paper was presented at the Association for Cognitive and Behavioral Therapies Convention on 15–18 November 2007 in Philadelphia, PA, USA.