No impact of Asian ethnicity on EORTC QLQ-C30 scores: Group differences and differential item functioning in paroxysmal nocturnal hemoglobinuria

BACKGROUND: Paroxysmal nocturnal hemoglobinuria (PNH) is a rare, life-threatening terminal-complement-mediated disease resulting in intravascular hemolysis and thrombosis with significant morbidity and premature mortality. There exists no disease-specific quality-of-life (QOL) measure for PNH. Its QOL effects resemble those of hematologic cancers, which supports the use of cancer-specific QOL measures in PNH clinical trials. The European Organisation for Research and Treatment of Cancer (EORTC) QLQ-C30 has published norms for many European and North American countries, but not for Asian countries. We investigated differences by Asian ethnicity in scores and item function on the EORTC QLQ-C30. METHODS: This secondary analysis focused on two non-inferiority PNH trials (301 and 302) comparing eculizumab and ravulizumab (n = 441). Analysis of covariance examined the main effect of Asian ethnicity on baseline EORTC QLQ-C30 scores, after adjusting for propensity scores encompassing trial, demographic and clinical factors. Mixed modeling of longitudinal data compared subscale scores in Asian vs. non-Asian patients, after propensity adjustment. Differential item function (DIF) was examined using ordinal regression models at baseline and longitudinally, to predict item score from total score, ethnicity, and their interaction to test for uniform DIF (significant main effect for Asian) and non-uniform DIF (significant Asian-by-total-score interaction). RESULTS: Of the 15 baseline domains, Asian patients scored slightly better on role and emotional functioning and slightly worse on constipation and diarrhea (0.22 < Cohen’s d < 0.36). In longitudinal models, Asians reported slightly higher appetite loss, diarrhea, and financial difficulties than non-Asians (R(2) increment < 0.0005). There was negligible uniform and non-uniform DIF, i.e., R(2) 0 to 0.018, far below Zumbo’s (1999) criterion of 0.13. On average there were larger differences from norms for Asians (mean = 0.05, sd = 0.44) than non-Asians (mean = -0.07, sd = 0.36), but the size and direction of the differences varied considerably by domain, age, and gender. CONCLUSION: When compared to norms, Asian patients showed no systematic biases. DIF results supported this finding. We conclude that Asian ethnicity does not impact interpretation of EORTC QLQ-C30 scores. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12955-021-01860-3.