Keywords: Rasch model, kidney cancer, symptoms, validation, interpretation, psychometrics
Objectives: The Functional Assessment of Cancer Therapy–Kidney Symptom Index–Disease Related Symptoms (FKSI-DRS) was developed to assess patients’ kidney cancer-related symptoms. The Rasch rating scale, a one-parameter-logistic item response model, may enhance FKSI-DRS interpretation and validate its measurement properties. Methods: We applied the Rasch model to FKSI-DRS data from a randomized phase 3 trial in which first-line sunitinib therapy showed superiority to interferon-alfa (IFN-a) in patients with metastatic renal cell carcinoma (mRCC). Of 750 enrolled patients, 668 patients completed the questionnaire on cycle 1 day 28, and were evaluated in the current study. The nine FKSI-DRS items were analyzed to enhance interpretation of the summary score using an item characteristic curve (ICC) that related score to probability of reporting specific symptoms. Results: The Rasch model fitted the FKSI-DRS well: 8/9 items had acceptable infit and outfit statistics (<1.5, >0.5); item difficulty spanned a wide range (-3.23–1.64 logits); and the five response categories performed adequately. The ICC offered enhanced interpretation of FKSI-DRS: for example, an FKSI-DRS score of 27 (mean baseline score for total sample) indicated a 47% chance of reporting ’no’ to ’lack of energy’ while a 2-point difference between sunitinib and IFN-a, averaged across all assessments (29 vs. 27), corresponded to sunitinib achieving a 28% increase (13% absolute difference) in the probability of reporting ’no’ to ’lack of energy’ (60% vs. 47%). Conclusions: Data suggest that the FKSI-DRS is an adequate measure of symptom status in mRCC patients. The Rasch model supports its validation and enhances its interpretation.