1613 Leung,etal:FunctionalindexesinPsA Personal non-commercial use only. The Journal of Rheumatology Copyright © 2008. All rights reserved. Comparison of 4 Functional Indexes in Psoriatic Arthritis with Axial or Peripheral Disease Subgroups Using Rasch Analyses YING-YING LEUNG, LAI-SHANTAM, EMILYWAI-LIN KUN, KWOK-WAH HO, and EDMUND KWOK-MING LI ABSTRACT. Objective. Rasch item response theory analysis is essential in evaluating measurement tools in spe- cific disease cohorts.We compared the performance of 4 functional indexes in patients with psoriat- ic arthritis (PsA) in axial or peripheral disease subgroups. Methods. Across-sectionalstudywasperformedinasinglecenter.Functionaloutcomesassessedby the Health Assessment Questionnaire (HAQ), Bath Ankylosing Spondylitis Functional Index (BASFI), Dougados Functional Index (FI), and the physical functioning scale of the Medical Outcome Study Short-form 36 (SF-36-PF) were analyzed by the Rasch model for item fit, item sep- aration, measurement span, and distribution properties. Patient subgroups with axial or peripheral disease were analyzed for differential item functioning (DIF). Results. One hundred eight patients with PsA were assessed. The 4 functional indexes were highly correlated with each other and moderately correlated with patients’ perception of health and pain scores. Floor effects were less marked in SF-36-PF. The 4 indexes satisfied the unidimensionality assumption of the Rasch model. HAQ and SF-36-PF had better information-weighted fit statistics (INFIT) and outlier-sensitive (OUTFIT) statistics. HAQ had the poorest item separation. SF-36-PF hadthehighestitem separation(6.99),reliability(0.85),andthelongestspanofitemthreshold(9.03 logits). Only 1 and 2 items in BASFI and Dougados-FI had DIF in patients with sacroiliitis. Conclusion. HAQ, BASFI, Dougados-FI, and SF-36-PF provide unidimensional measures of func- tional disability in PsA. SF-36-PF was the best in terms of less floor effect, highest item separation, longestspanofitemthreshold,andbetterdistributionalproperties.BASFIandDougados-FIbehaved similarly in patients with and without sacroiliitis and conferred no superiority in patients with axial disease. (First Release July 1 2008; J Rheumatol 2008;35:1613–21) KeyIndexingTerms: PSORIATICARTHRITIS FUNCTIONALASSESSMENT RASCHANALYSIS FromtheDepartmentofMedicineandGeriatrics,TaiPoHospital; DepartmentofMedicineandTherapeutics,PrincesofWalesHospital, ChineseUniversityofHongKong;andDepartmentofStatistics,Chinese UniversityofHongKong,HongKong,China. Y-Y.Leung,MD,ResidentSpecialist;L-S.Tam,MD,Departmentof MedicineandTherapeutics,PrincesofWalesHospital;E.W-L.Kun,MD; K-W.Ho,PhD,DepartmentofStatistics,ChineseUniversityofHong Kong;E.K-M.Li,MD,DepartmentofMedicineandTherapeutics,Princes ofWalesHospital. AddressreprintrequeststoDr.Y-Y.Leung,DepartmentofMedicineand Geriatrics,TaiPoHospital,HKSAR9ChuenOnRoad,TaiPo,NT, HongKongSAR.E-mail:katyccc@hotmail.com AcceptedforpublicationMarch23,2008. Psoriatic arthritis (PsA) is an inflammatory arthritis associ- ated with psoriasis. It affects young adults in their working ages, causing deformities, impaired quality of life, and poor physical function 1-3 . Functional disability is one of the major outcome domains used in randomized controlled tri- als, observational studies, and daily practice. The HealthAssessment Questionnaire (HAQ) was devel- oped for rheumatoid arthritis (RA) 4 .InPsA,HAQhasbeen used for measuring physical function in observational stud- iesandclinicaltrials 5,6 .HAQwasshowntobestronglycor- relatedtogripstrength,AmericanCollegeofRheumatology functional class, and fibromyalgia tender points, but was correlated only moderately to other measures of disease activity and poorly correlated with measures of spinal mobility. HAQ scores have been shown to be adequately sensitive to change after effective biologic therapies for peripheral joints in PsA 7,8 . A modification of the HAQ for spondyloarthropathies(HAQ-S) 9 was found to behave simi- larlytotheoriginalHAQ,eveninpeoplewiththeaxialpat- tern of disease. The HAQ has also been criticized for its floor effects, nonlinearity, and confusing items 10 . The Bath Ankylosing Spondylitis Functional Index (BASFI) has been validated and adopted by theAssessment inAS(ASAS)WorkingGroupformeasuringfunctionalout- come in ankylosing spondylitis (AS). It has been used in observational trials in spondyloarthropathies, which includ- ed patients with PsA with some evidence of validity 11 . It may be useful in PsA and may represent functional impair- mentasaresultofspinalinvolvementinthePsAcohort.The Medical Outcome Study Short-Form Health Survey (SF-36) is a generic measurement of quality of life. It has been vali- dated and used in epidemiology and treatment trials in PsA 1,7,8,12 . The 10 items on physical functioning (SF-36- PF) were extracted for analysis. The Dougados Functional