Assessing the severity of psychiatric disorders using the Health of the Nation Outcome Scales: An equipercentile linking analysis

Abstract

Objective: The Health of the Nation Outcome Scales was developed as an overall measure of mental health, applicable to the complete range of psychiatric disorders. Meanwhile, it is a benchmark tool for service providers and is also used for the allocation of costs and funding. The ability of the Health of the Nation Outcome Scales to assess and differentiate the severity of psychiatric disorders is largely unknown, as it is the interpretation of a change in score. We aim to establish Health of the Nation Outcome Scales cut-off and benchmark values for severity and improvement – respectively change, using equipercentile linking to the Clinical Global Impression scales. Methods: In a clinical sample of 30,616 individuals with a psychiatric disorder, we used a multivariate regression analysis to determine the correlation between the scales and possible confounders. We used an equipercentile linking analysis of the Clinical Global Impressions severity scale with the Health of the Nation Outcome Scales sum score to establish cut-off values for severity. The linking of the Health of the Nation Outcome Scales sum score difference and the percentage of change to the Clinical Global Impression improvement scale determined benchmark values for change (i.e. improvement or deterioration). Results: The Health of the Nation Outcome Scales and Clinical Global Impression scales showed a Spearman correlation of 0.38 ( p < 0.000). Clinical Global Impression–Severity: ‘borderline-ill’ corresponded to Health of the Nation Outcome Scales score 3–5; ‘mildly ill’ to 6–10; ‘moderately ill’ to 11–16; ‘markedly ill’ to 17–25; ‘severely ill’ to 26–35; and ‘extremely ill’ to a score ⩾36. The Spearman correlation between the percentage change of the Health of the Nation Outcome Scales was 0.39 ( p > 0.000); Clinical Global Impression–Improvement: ‘minimally improved’ corresponded to Health of the Nation Outcome Scales reduction of 4 points or 9%; ‘much-improved’ to 12 points or 48%; ‘very-much-improved’ to ⩾20 points or ⩾90%. Clinical Global Impression–Improvement: ‘minimally worse’ corresponded to an increase of 6 points or 25%; ‘much-worse’ to 12 points or 54%; and finally, ‘very-much-worse’ to >18 points or ⩾82%. Conclusion: Our results allow for the comparison of severity and response to treatment of patients with a psychiatric disorder, independent of diagnosis.