Thresholds for severity, remission and recovery using the functioning assessment short test (FAST) in bipolar disorder

Abstract

Background: Despite its importance, no distinction between none, mild, moderate and severe functional impairment is available. Categorization of functional impairment could help to better assess randomized controlled trials (RCT) and to study the correlates of functional impairment according to severity. The Functional Assessment Short Test (FAST) is one of the most widely used measures of functional impairment in bipolar disorder and related conditions, but to date no severity cut-offs have been determined for their use in clinical research and practice.

Method: FAST and Global Functioning Assessment (GAF) ratings from 65 euthymic outpatients with bipolar disorder at the Hospital Clínic in Barcelona were analyzed. A linear regression was computed using the FAST as the independent variable and the GAF as the dependent variable. Cut-offs scores for the FAST were estimated taking into account the GAF scores as a reference.

Results: Linear regression analysis with GAF scores as the dependent variable yielded the following equation: GAF _score = 91,41-1,031 * FAST _score. The cut-off scores for the FAST scale derived from this equation were as follows: scores from 0 to 11 included patients with no impairment. Scores from 12 to 20, represented the category of mild impairment. Moderate impairment comprised scores from 21 to 40. Finally, scores above 40 represent severe functional impairment. Further, the 4 × 4 cross-tabulation resulted in a significant association of FAST and GAF severity gradation: (Chi² = 95,095; df = 9; p < 0,001). Chance-corrected agreement was κ = 0,65 (p < 0.001).

Limitations: In the absence of a better alternative, the GAF, a broad clinical measure, was used as gold standard for establishing FAST categories according to severity.

Conclusion: The categorization of functional impairment in four categories based on empirical data shows that 12, 20 and 40 represent clinically meaningful cut-offs of the FAST for mild, moderate, and severe functional impairment and for functional recovery, remission, and improvement. The proposed categories are suitable to be further implemented in clinical studies and RCTs.