Thumboo, et al: Risk factors for PsA 757 From the Department of Health Sciences Research; Division of Rheumatology and Department of Dermatology, Mayo Clinic, Rochester, Minnesota, USA. Supported in part by grants from the NIH (AR-30582), USPHS, and the Ministry of Health, Singapore. J. Thumboo, FRCP(Edin); K. Uramoto, MD; M.I. Shbeeb, MD; W. M. O’Fallon, PhD; C.S. Crowson, BS; L.E. Gibson, MD; C.J. Michet Jr, MD, MPH; S.E. Gabriel, MD, MSc, Department of Health Sciences Research and Department of Dermatology. Address reprint requests to Dr. S.E. Gabriel, Department of Health Sciences Research, Mayo Clinic, 200 First Street SW, Rochester, Minnesota 55905. E-mail: gabriel.sherine@mayo.edu Submitted December 7, 2000; revision accepted October 22, 2001. Psoriatic arthritis (PsA) affects 5 to 8% of patients with psoriasis (PS) 1 and has been associated with substantial morbidity in some studies 2-4 . Factors associated with the development of PsA have largely been identified from case reports or case series, which are subject to bias and cannot establish a cause and effect relationship. Identifying factors that influence the development of PsA in patients with PS may provide insights into the etiology of PsA, and may lead to strategies to reduce the rate of PsA in patients with PS. We hypothesized that demographic, environmental, socio- economic, co-morbidity, disease or therapy related factors might influence the development of PsA, and studied the influence of these factors on the development and time to onset of PsA in an existing population based cohort of patients with PS in Olmsted County, Minnesota using a nested case control study design. MATERIALS AND METHODS Study design and population. Using the resources of the Rochester Epidemiology Project (REP), we conducted a nested case control study using a previously assembled database of all incident cases of PsA and all prevalent cases of PS resident in Olmsted County at diagnosis of PsA or PS from 1982 to 1991. The methods used to identify patients for inclusion in this database have been described 5 . The REP record linkage system contains information on virtually all medical care provided to residents of Olmsted County for as long as they reside in the county. Diagnoses made in clinics, hospitals, home visits, or at autopsy are entered into a centralized index, which enables the identification and retrieval of pertinent medical records. This system ensures virtually complete ascertainment and followup of all clinically defined disease among residents in Olmsted County. Matching of cases and controls. Of the 66 incident PsA cases identified through our previous epidemiologic studies, 6 were excluded from this analysis because they developed PsA before PS. Each remaining case was matched with 2 controls from the same cohort. Controls were selected from all PS patients without PsA who fulfilled the following matching criteria applied in sequence: case duration of PS at onset of PsA (± 2 yrs), age of case (± 2 yrs) at onset of PsA, gender, date of onset of PS (± 2 yrs) and Mayo Clinic registration number, which controlled approximately for opportunity of care in Olmsted county (Table 1). Risk Factors for the Development of Psoriatic Arthritis: a Population Based Nested Case Control Study JULIAN THUMBOO, KRISTINE URAMOTO, MOHAMMED I. SHBEEB, W. MICHAEL O’FALLON, CYNTHIA S. CROWSON, LAWRENCE E. GIBSON, CLEMENT J. MICHET Jr, SHERINE E. GABRIEL ABSTRACT Objective. To identify factors influencing the development of psoriatic arthritis (PsA) in a popula- tion-based, inception cohort of psoriasis (PS) patients. Methods. Using the population-based data resources of the Rochester Epidemiology Project, which ensures virtually complete ascertainment of all clinically defined conditions, we previously identi- fied all incident cases of PsA and prevalent cases with PS from 1/1/1982 to 12/21/1991. In this nested case-control study, we assessed potential factors influencing the development of PsA in this cohort using medical record and patient survey information. Each case of PsA was matched with 2 PS controls on age, gender and PS duration/date of onset. Factors influencing the development of PsA were identified, adjusting for the influence of other variables using conditional logistic regres- sion for medical record data and logistic regression for survey data. Results. Sixty incident PsA cases were matched with 120 controls with PS. The median age at onset of PS was 31.7 (3.0–78.3) years, and 49% of subjects were male. There were 67% (n = 40) survey responders among cases and 48% (n = 58) among controls. Corticosteroids were used by 10 cases and 6 controls in the 2 years prior to onset of PS through to the development of PsA, and increased the risk of developing PsA (odds ratio 4.33, 95% CI = 1.34–14.02). Pregnancy occurred in 2 cases and 12 controls in the same period, and decreased the risk of developing PsA (odds ratio 0.19, 95% CI = 0.04–0.95). These associations remained significant after adjusting for the influence of gender, age, and duration of psoriasis. Conclusion. Corticosteroid use and pregnancy, both of which modulate the immune response, may influence the development of PsA in patients with PS. (J Rheumatol 2002;29:757–62) Key Indexing Terms: PSORIATIC ARTHRITIS RISK FACTORS GLUCOCORTICOIDS PREGNANCY CASE-CONTROL STUDIES Personal non-commercial use only. The Journal of Rheumatology Copyright © 2002. 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