The Hunter Syndrome-Functional Outcomes for Clinical Understanding Scale (HS-FOCUS) Questionnaire: evaluation of measurement properties

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).

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.

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.

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.