180 Chapter 7 to justify the use of these disease-specific PROMs in daily clinical practice [20]. For these reasons, standardization of outcomes and PROM administration in hemophilia care and research is essential, as described by Van Hoorn et al. [11,15,20,23]. Several initiatives have recently worked on core outcome sets for patients with hemophilia [24-26], resulting in the patient-reported outcomes measurement information system (PROMIS) being selected as one of the included measurement tools [26]. PROMIS provides a set of person-centered, standardized instruments to measure a broad range of health domains (physical, mental, and social health) in children [27,28]. In contrast to legacy instruments that are based on the Classical Test Theory (CTT), PROMIS measures were developed according to the Item Response Theory (IRT) [29,30]. An important advantage of the use of IRT is the option of Computerized Adaptive Testing (CAT) [28,29]. With CAT, items are offered to patients based on their previous answers. Consequently, PROMIS measures are shorter, items are more tailored to the patients’ situation, and the measurement is more reliable in comparison to existing PROMs [28,30]. Recent studies showed that, in (young) adult patients with hemophilia, PROMIS measures are effective, reliable, and valid with low floor- and ceiling effects [15,31-33]. However, it is unclear if PROMIS instruments are also suitable for children with hemophilia. Therefore, the aim of this study is to evaluate the feasibility, measurement properties, and outcomes of 8 relevant PROMIS pediatric measures for boys with hemophilia. 2. Methods 2.1. Study population and procedure All boys (8-17 years) treated for mild to severe hemophilia A or B in one of the hemophilia treatment centers in the Netherlands (Amsterdam University Medical Centers, Van Creveldkliniek, Erasmus University Medical Center, Radboud University Medical Center, or the University Medical Center Groningen) were eligible to participate in this multicenter study. Between June 2021 and December 2021, patients were invited to participate by email and received a personal link to the study website (https://promis.hetklikt.nu/hemofilie/) of the KLIK PROM portal [34]. Caregivers were asked to complete a sociodemographic questionnaire, and children were asked to complete 8 PROMIS instruments and 2 legacy instruments (Haemophilia Quality of Life Questionnaire for Children (Haemo-QoL) and Pediatric Hemophilia Activities List (PedHAL)). Children with insufficient knowledge of the
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