Comparing illness duration and age as predictors of treatment outcome in female inpatients with anorexia nervosa

ABSTRACT

It has been widely assumed that longer illness duration predicts poorer treatment outcome in persons with anorexia nervosa (AN). However, studies on the prognostic effects of illness duration have produced mixed results. Thus, the aim of the current study was to examine the relationship between illness duration and short-term treatment outcome in a large sample of female inpatients with AN (n = 902, aged 12–73 years). Treatment outcome variables included body mass index, therapist-rated global functioning (Global Assessment of Functioning scale and Clinical Global Impression–Improvement scale) and subscales of the Eating Disorder Inventory–2. Longer illness duration predicted smaller weight gain, smaller improvements in global functioning, and smaller decreases in self-reported eating disorder symptoms. However, illness duration was almost perfectly correlated with patients’ age (r = .81, 95% CI [.76, .85]), and comparing regression models revealed that models using either illness duration or age were indistinguishable. Results suggest that longer illness duration does indeed relate to worse short-term treatment outcome in inpatients with AN. This effect, however, does not add significant information above and beyond patients’ age and, thus, the importance of illness duration for anticipating treatment outcome both in research and in clinical practice must be critically examined.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data set and annotated R-code for our main analyses are available at https://osf.io/9hsvq.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/10640266.2022.2114586

Additional information

Funding

The author(s) reported that there is no funding associated with the work featured in this article.