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.