The Hunter syndrome-functional outcomes for clinical understanding scale (HS-FOCUS) questionnaire: evaluation of measurement properties.

PURPOSE: This study was to conduct the psychometric validation of the patient and parent versions of the Hunter syndrome-functional outcomes for clinical understanding scale (HS-FOCUS). METHODS: Data collected in a 53-week placebo-controlled multinational trial were used to evaluate item performance and reliability, validity, and ability to detect change of the six HS-FOCUS function domains. RESULTS: HS-FOCUS was completed by 49 patients above 12 years old and 84 parents. Floor effects and high average inter-item correlations suggested that some items were less informative or redundant. For both patients and parents, the internal consistency and test-retest reliability met the >0.70 criteria for all domains except for the breathing, sleeping, and schooling/work in patients. Construct validity showed moderate to high correlations with CHAQ, CHQ, and HUI3 in activity-related concepts. Significant differences in domain scores were found in most domains among severity in disability measured by CHAQ DIS. Significant differences in HS-FOCUS change scores were found in patients whose CHAQ DIS score also changed. CONCLUSIONS: Psychometric validation of the HS-FOCUS demonstrates it is a reliable, valid, and responsive instrument that can be applied in clinical trials or disease registries. Findings on the individual item performance suggest some items could be removed without compromising its validity.