Knee osteoarthritis (OA) is a major cause of joint dysfunction, pain and disability. In 1996 we recruited a cohort of patients with predominantly medial tibiofemoral OA of the knee to monitor the natural progression of the disease sequentially over 5 years and to test the hypothesis that progression of OA is episodic or phasic. We report here the analysis of longitudinal measurements of serum cartilage oligomeric matrix protein (COMP) in relation to disease outcome at the end of the study. Acknowledgements: This study was supported by the Arthritis and Rheumatism Council, UK; Hoffmann-La Roche Ltd, Roche Diagnostics, Switzerland; and the NHS Executive South West Research and Development Directorate, Bristol, UK. We are grateful to AnaMar Medical for measuring the serum COMP concentrations. LONGITUDINAL STUDY OF SERUM CARTILAGE OLIGOMERIC MARTIX PROTEIN (COMP) AND PHASIC PROGRESSION OF KNEE OSTEOARTHRITIS M Sharif 1, JR Kirwan 2, RGranell 2,3, S Clarke 2 1 Dept of Anatomy, 2 Rheumatology Unit, 3 Dept of Mathematics and 4 Dept of Pathology & Microbiology University of Bristol, UK Complete data was available on 115/135 patients of which 37 progressed (22 by TKR and 15 by TFJ2mm). The mean (SD) ages of the progressors and non-progressors were 64.2 (7.8) years and 63.3 (10.6) year, and proportion of females 51% and 56% respectively. Baseline [COMP] was significantly higher in the progressors compared to the non-progressors (Figure 1). The probability of progression increases with increasing serum concentrations of COMP (Figure 2). 7/37 patients progressed between 0–2 years and again 3-5 years by the same criteria. In these patients serum COMP was higher during both periods of progression. Elevated COMP concentrations are associated with progression of knee OA. Ordered logistic regression analysis has shown that for each unit rise in serum COMP the probability of progression increases by 15%. The patterns of progression seen in some of the patients are consistent with the hypothesis that progression of knee OA is episodic or phasic. Figure 1: Serum COMP concentrations at entry to the study in relation to disease outcome at 5 year follow up. The horizontal bars indicate the mean in each group. RESULTS BACKGROUND METHODS Patients with knee pain and radiographic evidence of OA had clinical examination and blood samples at 0, 6, 12, 18, 24, 30, 36, 42, 48 and 60 months. Plain knee x-rays were taken at 0, 2, 3 and 5 years and read blind using standard techniques OA progression was defined by previously used criteria (reduction of joint space by at least 2mm in the tibiofemoral joint space [TFJ2 mm] or total knee replacement [TKR]). Serum COMP was measured using a COMP ELISA kit (AnaMar Medical, Lund, Sweden). Ordered logistic regression modeling was performed to determine the probability of progression in relation to serum COMP concentrations [COMP]. CONCLUSIONS Federation of European Connective Tissue Societies, Taormina-Giardini Naxos, Sicily, 9-13th July 2004.