The study aimed to examine agreement between patients' and professional staff members' ratings on the Global Assessment of Functioning scale (GAF).

A total of 191 young adult psychiatric outpatients were included in a naturalistic, longitudinal study. Axis I and axis II disorders were assessed by means of the Structured Clinical Interview for DSM-IV. Before and after treatment, patients and trained staff members did a GAF rating. Agreement between GAF ratings was analyzed using the intra-class correlation coefficient (ICC).

The overall intra-class correlation coefficients before and after treatment were 0.65 and 0.86, respectively. Agreement in different axis I diagnostic groups varied, but was generally lower before treatment as compared to after treatment (0.50–0.66 and 0.78–0.90, respectively). Excessive psychiatric co-morbidity was associated with the lowest inter-rater reliability. Agreement, with respect to change in GAF scores during treatment, was good to excellent in all groups.

Overall, agreement between patients' and professionals' ratings on the GAF scale was good before and excellent after treatment. The results support the usefulness of the self-report GAF instrument for measuring outcome in psychiatric care. However, more research is needed about the difficulties in rating severely disordered patients.