BMI Threshold for AN — MRCPsych Paper B MCQ

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EasyEating DisordersBMI Threshold for ANMRCPsych Paper B

A 24-year-old woman is assessed for restrictive eating, fear of weight gain and overvaluation of thinness. Under current ICD-11 diagnostic requirements for anorexia nervosa, which body mass index is taken as the threshold for "significantly low body weight" in an adult?

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Correct answer: E — Below 18.5 kg/m²

Explanation lettering: E = shown as A · C = shown as B · A = shown as C · B = shown as E

B is correct. In current classification (ICD-11, mirrored by DSM-5-TR), the adult marker of significantly low body weight is BMI below 18.5 kg/m²; this cut-off is what separates anorexia nervosa from atypical anorexia nervosa, where weight remains within or above the normal range. Low BMI alone is never sufficient: restrictive or weight-lowering behaviours plus weight/shape psychopathology are required, and rapid substantial weight loss can be significant at a higher BMI. C (17.5 kg/m²) is the superseded ICD-10 figure and is the commonest error. D (15 kg/m²) and E (12 kg/m²) indicate severe or life-threatening underweight relevant to medical risk assessment and admission decisions, not to the diagnostic threshold. A (20 kg/m²) lies within the healthy range. NICE NG69 warns against using BMI alone to decide whether to offer treatment.

Reference: Harrop EN et al. / Ide M et al. — 'Atypical anorexia nervosa: Implications of clinical features and BMI cutoffs' (PubMed 36740848), 2023: BMI cut-off 18.5 kg/m² distinguishes anorexia nervosa from atypical anorexia nervosa. https://pubmed.ncbi.nlm.nih.gov/36740848/