Equipercentile linking of scales measuring functioning and symptoms: examining the GAF, SOFAS, CGI-S, and PANSS

The Global Assessment of Functioning (GAF) and the Social and Occupational Functioning Assessment Scale (SOFAS) are rating scales commonly used to assess the level of functioning in patients with schizophrenia. To understand the correspondence of scores between GAF and SOFAS, and what they mean from a clinical point of view, we examined the linkage of (a) GAF with SOFAS total scores, (b) GAF with Clinical Global Impressions Scale (CGI) and Positive and Negative Syndrome Scale (PANSS), and (c) SOFAS with CGI and PANSS. We used the equipercentile linking method to identify corresponding scores of simultaneous GAF, SOFAS, PANSS and CGI ratings in 1208 patients from a naturalistic European cohort study. Data were collected at baseline and at months 6, 12, 18 and 24. GAF and SOFAS total scores were found to be practically exchangeable. Both scales had strong negative correlations with CGI and PANSS; the linkage also suggested the presence of slight impairment in functioning even when patients are free from symptoms. These findings are important for the comparison of scores when different rating scales are used. We present a detailed conversion table in an online supplement.