Internal Psychometric Validation of an International Burden of Illness Survey for Idiopathic Multicentric Castleman Disease.

Introduction: Idiopathic multicentric Castleman disease (iMCD) is a rare, chronic, debilitating lymphoproliferative disorder where the mainstay of treatment is symptom management. Our recent international patient survey showed that patients with iMCD have a high symptom burden that has a significant negative patient-reported impact on several aspects of daily life. As part of our ongoing work towards the development of an iMCD symptom burden scale, assessing the survey's psychometric properties is a critical step in understanding its adequacy, relevance, and usefulness. As iMCD is a rare disease, there are challenges to conducting such psychometric analyses which we describe.

Methods: As part of the exploratory psychometric analysis, three a priori hypothesis sets (HS) were generated by interviewing an iMCD-experienced clinician, a patient, and a caregiver to explore the iMCD patient survey's internal construct validity, given no gold standard iMCD measure exists for external construct validation. HS-1 hypothesized that a convergent or discriminant relationship exists with the patients' self-assessment of symptom effect on daily life between two potentially related or unrelated symptoms, respectively. HS-2 hypothesized that having a greater number of symptoms has a positive convergent relationship with the patients' assessment of symptoms' effect on daily life. Finally, HS-3 hypothesized that patients receiving treatment versus no treatment was associated with patients reporting less effect of symptom burden on their daily life. Spearman's rank absolute correlation strength (ACS) was used for HS-1 and HS-2 (convergent relationship, ACS ≥ 0.3 and p value < 0.05; divergent relationship, ACS < 0.3), and Cohen's d to quantify standardized absolute effect sizes (AES) for HS-3 (AES ≥ 0.5 and p value < 0.05).

Results: Our analyses partially supported HS-1. None of the three positive convergent relationships were supported. Of the six discriminant relationships, only dizziness with impaired cognitive function and tiredness with dizziness were supported. HS-2 analyses showed there was convergent validity between the number of symptoms and their effect on aspects of daily life. HS-3 analyses did not provide evidence to support the hypothesis.

Conclusion: These internal psychometric construct analyses provide initial support for the bespoke iMCD patient survey and will guide additional work towards the development of the first iMCD-specific symptom burden scale.