How to Define and Report Dropout in Blended Therapy for Mental Health: A Scoping Review and Comparative Secondary Analysis of Operational Definitions.

Background. Evidence suggests that blended therapy, which combines face-to-face psychotherapy with digital components, has the potential to reduce treatment dropout rates. However, there is inconsistency in how treatment dropout is defined in the blended therapy context. The combination of face-to-face and digital components adds complexity to potential definitions of dropout: dropout may involve discontinuing in-person sessions, disengaging from digital components, or both. Therefore, this study aimed to (1) identify operational definitions of dropout in blended therapy through a scoping review and (2) examine how different dropout definitions affect dropout rates, treatment outcomes, and usage patterns using data from a large blended care trial (PsyTOM trial). Methods. For aim 1, in January 2025, we first conducted a scoping review to identify existing operationalizations of treatment dropout. For aim 2, after synthesizing the results, we applied five resulting operational definitions for treatment dropout to the PsyTOM trial data. We assessed the different dropout rates per definition, their association with depressive symptoms, anxiety, and quality of life, and distinct treatment usage patterns via cluster analysis.Results. The literature search identified 14 studies providing operational treatment dropout definitions. After synthesizing, applying five definitions to the PsyTOM dataset revealed substantial variation in dropout rates (up to 25%) and their associations with treatment outcomes. For some definitions, there were significant group differences between participants who dropped out and those who completed treatment in associations with treatment outcomes, for other definitions, these differences were only descriptive. Cluster analysis identified four usage patterns, with logistic regression indicating that Cluster 2 (average users) had a significantly lower dropout risk than Cluster 1 (minimal users) (OR = 0.25).Conclusion. Treatment dropout definitions in blended therapy are highly heterogeneous, impacting how dropout rates and associations with outcomes are interpreted. To enhance comparability across studies, we recommend reporting dropout definitions explicitly, distinguishing between digital vs. face-to-face dropout rates, and including dropout timing and symptom tracking in blended therapy research.