VALIDATION OF THE PHENYLKETONURIA-SYMPTOM SEVERITY AND IMPACTS SCALE (PKU-SSIS) IN ADULTS WITH PKU
Author(s)
Hafiz Oko-osi, MS, PharmD1, Amy Howerter, PhD2, Ibrahim Alfayoumi, MS2, Tanu Bhargava, PhD2, Kate Delaney, BA, CSP1, Karon Cook, PhD2, Ebony Dashiell-Aje, PhD1, David A. Andrae, PhD1;
1BioMarin Pharmaceutical Inc., Novato, CA, USA, 2Clinical Outcomes Solutions, Tucson, AZ, USA
1BioMarin Pharmaceutical Inc., Novato, CA, USA, 2Clinical Outcomes Solutions, Tucson, AZ, USA
OBJECTIVES: Phenylketonuria (PKU), a rare metabolic genetic disorder caused by a deficiency of phenylalanine hydroxylase, can cause cognitive, psychological, and emotional symptoms that often affects patient quality of life. To support new treatments and disease management, the Phenylketonuria Symptom Severity and Impacts Scale (PKU-SSIS) was developed as a disease-specific patient-reported outcome measure to understand the adult PKU symptom experience and impacts on everyday life. This study conducted a psychometric assessment of the PKU-SSIS Adult version to determine its suitability as a fit-for-purpose measure in adults with PKU.
METHODS: A non-interventional psychometric evaluation study of the PKU-SSIS Adult version (18+ years) was completed using an online survey at two timepoints. Participants were recruited across three countries through healthcare professionals, PKU advocacy groups, third-party recruiters, and PKU clinics. Additional reference measures were included as part of the psychometric evaluation.
RESULTS: Adults with PKU (n=109) completed two surveys two weeks apart. Item level analyses revealed several poorly performing items and skewed distributions. Multiple models were tested to examine the response options and factor structure, and a binary unidimensional model was explored. After removing misfitting items and collapsing response options to a dichotomous scale, a unidimensional graded response model demonstrated good fit (ie, RMSEA = 0.07, SRMSR = 0.08). This structure aligned with observed participant response patterns, characterized by strong endorsement of higher or lower response categories and infrequent selection of middle categories. This model was used to construct a total score that was applied to the psychometric evaluation. Psychometric evaluation revealed the PKU-SSIS total score demonstrates good reliability (KR-20 =0.88; ICC(2,1)=0.82 - 0.88) and strong convergent validity with measures of cognitive and emotional function (r= 0.72 - 0.74).
CONCLUSIONS: Psychometric evidence for the PKU-SSIS in adults with PKU is strong, supporting the use of the PKU-SSIS for assessing symptoms and impacts of PKU in research and practice.
METHODS: A non-interventional psychometric evaluation study of the PKU-SSIS Adult version (18+ years) was completed using an online survey at two timepoints. Participants were recruited across three countries through healthcare professionals, PKU advocacy groups, third-party recruiters, and PKU clinics. Additional reference measures were included as part of the psychometric evaluation.
RESULTS: Adults with PKU (n=109) completed two surveys two weeks apart. Item level analyses revealed several poorly performing items and skewed distributions. Multiple models were tested to examine the response options and factor structure, and a binary unidimensional model was explored. After removing misfitting items and collapsing response options to a dichotomous scale, a unidimensional graded response model demonstrated good fit (ie, RMSEA = 0.07, SRMSR = 0.08). This structure aligned with observed participant response patterns, characterized by strong endorsement of higher or lower response categories and infrequent selection of middle categories. This model was used to construct a total score that was applied to the psychometric evaluation. Psychometric evaluation revealed the PKU-SSIS total score demonstrates good reliability (KR-20 =0.88; ICC(2,1)=0.82 - 0.88) and strong convergent validity with measures of cognitive and emotional function (r= 0.72 - 0.74).
CONCLUSIONS: Psychometric evidence for the PKU-SSIS in adults with PKU is strong, supporting the use of the PKU-SSIS for assessing symptoms and impacts of PKU in research and practice.
Conference/Value in Health Info
2026-05, ISPOR 2026, Philadelphia, PA, USA
Value in Health, Volume 29, Issue S6
Code
PCR188
Topic
Patient-Centered Research
Topic Subcategory
Patient-reported Outcomes & Quality of Life Outcomes
Disease
No Additional Disease & Conditions/Specialized Treatment Areas, SDC: Rare & Orphan Diseases