Thresholds for Severity, Remission and Recovery Using the Functioning Assessment Short Test (FAST) in Bipolar Disorder

Recovery of functioning is an important construct to measure when assessing treatment efficacy for interventions in bipolar disorder.¹ The FAST is a tool for measuring functional outcome in patients with bipolar disorder, but it lacks psychometric data on cutoffs to indicate severity of functional impairment.¹ Severity level cutoffs could be useful for determining the efficacy of pharmacologic and nonpharmacologic treatments in randomized controlled trials.

Why was the research needed?

Randomized controlled trials rely on clinician ratings, such as ratings of functioning, to assess treatment efficacy.¹ Despite the FAST scale being validated for the assessment of functioning in patients with bipolar disorder,¹ to our knowledge no study has yet provided validated cutoff scores to help researchers and clinicians determine the severity of a patient’s functional impairment for all types of functioning. Patients with bipolar disorder vary in their functioning beyond “impaired” or “not impaired”, and functional level is a meaningful endpoint for assessing treatment response, so accurately capturing gradients of functioning in this population is needed.1

What did the researchers do?

The authors analyzed mean total scores on the FAST and the Global Assessment of Functioning-Functioning (GAF-F) subscale from 65 euthymic patients with bipolar I disorder or bipolar II disorder in order to determine impairment levels.¹ Pearson correlations were used to measure the relationship between FAST and GAF-F scores. A linear regression was performed with FAST as the independent variable and the GAF-F as the dependent variable. FAST cutoffs were calculated based on the GAF-F cutoffs (ie, no impairment, mild impairment, moderate impairment, and severe impairment).¹ Once FAST cutoffs were calculated, cross tabulations, Chi-square tests, and Cohen’s kappa were employed to measure reliability.

What were the main results of the study?

The authors identified GAF-F ranges that corresponded to FAST ranges for no impairment and for mild, mild-to-moderate, moderate, moderate-to-severe, and severe impairment, as shown in the table below.

Cross-tabulations indicated the severity ranges for the FAST and GAF-F were significantly correlated (p < 0.001), and a Cohen’s kappa of 0.65 (p<0.001). The cutoffs correctly classified 76.9% of all patients (n=50) and correctly classified 81.5% of patients with no impairment (n=22). Of patients with moderate impairment, the cutoffs correctly classified 90.5% of patients (n=19). Two-thirds of the patients with severe impairment (66.7%, n=4) were correctly classified. The cutoffs correctly classified the least number of patients with mild impairment, at 45.5% (n=5).¹

A second cross-tabulation was performed on euthymic and noneuthymic patients (n=101) and found similar patterns. Among all patients, 81.2% were classified correctly. Nonimpaired patients were correctly classified 82.1% of the time, moderately impaired patients 93.8% of the time, and severely impaired patients 79.3% of the time. The lowest number of patients correctly classified again in the second cross-tabulation, at 50%, were mildly impaired.¹

Using these validated cutoffs, the authors recommended the FAST cutoff scores for severity shown in the table below.

Using these validated FAST cutoff scores, the original sample of euthymic patients was classified as 44% unimpaired, 15% mildly impaired, 35% moderately impaired, and 6% severely impaired using the validated FAST cutoff levels.¹

Why are these results potentially important?

Given that existing functional impairment assessments largely classify patients with bipolar disorder as either impaired or nonimpaired, ignoring gradients of severity, results from this analysis offer clinicians an approach to measuring patient functioning via the FAST.¹ Indeed, these results suggest FAST cutoffs can give clinicians a way to satisfactorily demarcate nonimpaired, mildly impaired, moderately impaired, and severely impaired patients.¹ Additionally, that FAST and GAF-F scores were correlated with one another means outcomes from studies using the FAST or the GAF-F to capture functioning in bipolar disorder can be compared to one another.¹

These results also potentially carry importance for randomized controlled trials. FAST severity cutoffs can ensure patients enrolled in bipolar disorder clinical trials are truly impaired, making it more likely that researchers will be able to detect the clinical effect of a drug or psychological intervention on patient functioning.1 This approach could make functional outcome reporting in patients with bipolar disorder more useful to clinicians.¹

These findings should be considered in light of some limitations. In this study, the mild impairment classification had the weakest reliability, even after increasing the same size from 65 to 101. Larger samples sizes would be needed to determine whether reliability improves as well as to determine whether reliability ratings in the other classifications can be replicated. The GAF-F itself does not have established cutoff scores, and those used in the study were based on the authors’ expertise.¹ Further, the study’s use of a small number of patients is also a limitation.¹

What’s next?

Ongoing research is also needed to assess FAST cutoffs in the context of medication use and comorbidities, as each variable could impact functional outcomes in patients with bipolar disorder.¹ Additional studies are also needed to replicate and validate findings from this study.

This summary was prepared independently of the study’s authors. The content presented here is provided for educational purposes only. It is not intended as, nor is it a substitute for medical care or advice. Healthcare professionals should use their clinical judgment when reviewing educational resources on NP Psych Navigator.