District Hospital Trauma Mental Health Screening Nurse-Led Brief Intervention Fidelity Drift and Post-Traumatic Stress Disorder Symptom Non-Response Among Kenyan Survivors: A Multilevel Variance Decomposition of Therapeutic Drift Thresholds

Nurse-led brief interventions for post-traumatic stress disorder (PTSD) in Kenyan district hospitals are vulnerable to fidelity drift, the gradual erosion of protocol adherence across sessions and providers. The framework treats fidelity as a time-varying, hierarchically nested construct and specifies a threshold model in which the probability of non-response is a function of cumulative drift exposure. Using a conceptual parameterisation grounded in implementation science and trauma-focused cognitive behavioural therapy, the paper derives the variance components, defines the drift threshold estimand, and proposes a three-level logistic model with random intercepts and random slopes. The framework is illustrated with a hypothetical calibration scenario to demonstrate how the threshold can be identified and interpreted. The contribution is a formal, testable specification of therapeutic drift thresholds that can guide fidelity monitoring in resource-constrained trauma care settings, where routine outcome monitoring is feasible but session-level fidelity data are sparse.