Test-retest reliability and measurement error of the pain interference questionnaire for cerebral palsy and the fear of pain questionnaire adapted for cerebral palsy

Background The Pain Interference Questionnaire for Cerebral Palsy (PIQ-CP) and Fear of Pain Questionnaire adapted for Cerebral Palsy (FOPQ-CP) are two patient-reported outcome measures (PROMs) developed and validated for children and young people with cerebral palsy and chronic pain, with specific adaptations to improve self-report opportunity. Test-retest reliability is an important psychometric property to ensure PROMs are used accurately to assess change in intervention and longitudinal studies. This study aimed to determine the test-retest reliability of the PIQ-CP and FOPQ-CP in children and young people with cerebral palsy. Methodology Children and young people with CP aged 5–30 years with a history of ongoing pain were recruited from four Australian sites within a larger cross-sectional study. Participants completed the PIQ-CP or FOPQ-CP on two occasions 2–4 weeks apart. Test-retest reliability and measurement error were assessed using intraclass correlation coefficients (ICC absolute agreement), standard error of measurement (SEM) and smallest detectable change (SDC). Results Test–retest reliability was moderate for the FOPQ-CP (Pain-related fear (subscale 1): ICC = 0.74 [0.58–0.85], SEM = 2.12, SDC = 5.88; n = 44; Pain-related avoidance (subscale 2): ICC = 0.62 [0.40–0.77], SEM = 4.07, SDC = 11.27; n = 44) and for the Pain Interference Questionnaire – Cerebral Palsy (PIQ-CP) (Pain interference: ICC = 0.71 [0.53–0.83], SEM = 6.29, SDC = 17.43; n = 45). The mean interval between assessments was 26.8 days. Conclusions The PIQ-CP and FOPQ-CP have demonstrated sufficient test–retest reliability for children and young people with cerebral palsy, including for those with diverse abilities. They can now be used in clinical and research settings to prioritise self-reported pain interference and pain-related fear/avoidance in intervention studies.