The development, content and response process validation of a caregiver-reported severity measure for CDKL5 deficiency disorder

Background: CDKL5 Deficiency Disorder (CDD) is a severe X-linked developmental and epileptic encephalopathy. Existing developmental outcome measures have floor effects and cannot capture incremental changes in symptoms. We modified the caregiver portion of a CDD clinical severity assessment (CCSA) and assessed content and response-process validity. Methods: We conducted cognitive interviews with 15 parent caregivers of 1-39-year-old children with CDD. Caregivers discussed their understanding and concerns regarding appropriateness of both questions and answer options. Item wording and questionnaire structure were adjusted iteratively to ensure questions were understood as intended. Results: The CCSA was refined during three rounds of cognitive interviews into two measures: (1) the CDD Developmental Questionnaire - Caregiver (CDQ-Caregiver) focused on developmental skills, and (2) the CDD Clinical Severity Assessment - Caregiver (CCSA-Caregiver) focused on symptom severity. Branching logic was used to ensure questions were age and skill appropriate. Initial pilot data (n = 11) suggested no floor effects.