title: Using the Rasch model to validate and enhance the interpretation of the Functional Assessment of Cancer Therapy-Kidney Symptom Index--Disease-Related Symptoms scale.
abstract: OBJECTIVE
  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 in patients with metastatic renal cell carcinoma. 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 by using an item characteristic curve that related score to probability of reporting specific symptoms.
  
  
  RESULTS
  The Rasch model fitted the FKSI-DRS well: 8 of 9 items had acceptable infit and outfit statistics (<1.5, >0.5); item difficulty spanned a wide range (-3.23 to 1.64 logits); and the five response categories performed adequately. The item characteristic curve 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," although a two-point difference between sunitinib and interferon-alfa, 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 patients with metastatic renal cell carcinoma. The Rasch model supports its validation and enhances its interpretation.
authors.affiliation: Global Outcomes Research, Pfizer Inc, New London, CT 06320, USA. xuemei.luo@pfizer.com
authors.forename: Xuemei
authors.initials: X
authors.lastname: Luo
authors.forename: Joseph C
authors.initials: JC
authors.lastname: Cappelleri
authors.forename: David
authors.initials: D
authors.lastname: Cella
authors.forename: Jim Z
authors.initials: JZ
authors.lastname: Li
authors.forename: Claudie
authors.initials: C
authors.lastname: Charbonneau
authors.forename: Sindy T
authors.initials: ST
authors.lastname: Kim
authors.forename: Isan
authors.initials: I
authors.lastname: Chen
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Motzer
mesh_terms: D000970:Antineoplastic Agents / Q000009:adverse effects / Q000627:therapeutic use; D002292:Carcinoma, Renal Cell / Q000188:drug therapy*; D003627:Data Interpretation, Statistical; D005260:Female; D006305:Health Status Indicators; D006801:Humans; D007155:Immunologic Factors / Q000009:adverse effects / Q000627:therapeutic use; D007211:Indoles / Q000009:adverse effects / Q000627:therapeutic use; D016898:Interferon-alpha / Q000627:therapeutic use; D007680:Kidney Neoplasms / Q000188:drug therapy*; D008297:Male; D008875:Middle Aged; D018803:Models, Economic; D015233:Models, Statistical; D011758:Pyrroles / Q000009:adverse effects / Q000627:therapeutic use; D000077210:Sunitinib; D011795:Surveys and Questionnaires
chemical_list: D000970:Antineoplastic Agents; D007155:Immunologic Factors; D007211:Indoles; D016898:Interferon-alpha; D011758:Pyrroles; D000077210:Sunitinib