44 CHAPTER 3 body positive (anti-cyclic citrullinated peptide test >25 kAU/l and/or IgM-rheumatoid factor >12.5 kU/l). Two patients (patients 3 and 11) were treated with a biological at the time of arthroscopy, i.e. rituximab and infliximab (both last infusion 1 month before sampling). More details on patient characteristics are shown in Table 1. From 7 patients synovial tissue biopsies were taken from two inflamed joints, all paired with peripheral blood. From 4 of these patients, we also collected synovial fluid from the biopsied joint prior to the arthroscopy. Three additional patients were included for paired SF and PB analysis. The study was approved by the independent Medical Ethics Committee of the Amsterdam University Medical Center/University of Amsterdam and performed according to the Declaration of Helsinki. All patients gave written informed consent. Table 1 | Patient characteristics (n=11) Age (mean (SD)), years 53 (13) Male (n (%)) 2 (18%) IgM-RF positive (n (%)) 11 (100%) IgM RF level (median (IQR)), kU/L 68 (33-322) ACPA positive (n (%)) 6 (55%) ACPA level (median (IQR)), kAU/L 84 (8-499) IgM-RF and ACPA both pos. (n (%)) 6 (55%) Disease duration (median (IQR)), months 45 (3-192) DAS28 (mean (SD)) 5.2 (1.2) Patients without therapy (n (%)) 7 (64%) Patients treated with csDMARD (n (%)) 4 (36%) Patients treated with bDMARD (n(%)) 2 (18%) IgM-RF, IgM rheumatoid factor; ACPA, anti-citrullinated peptide antibodies; csDMARD, conventional synthetic disease-modifying antirheumatic drugs; bDMARD, biological disease-modifying antirheumatic drugs. Sampling of synovial biopsies, synovial fluid and peripheral blood To obtain ST biopsies a minimally invasive arthroscopy was performed from a clinically inflamed knee or ankle, as described previously [13]. From 12 out of the 15 inflamed knee joints biopsied, ST biopsies were taken from two locations, the infrapatellar (IP) and the suprapatellar (SP) region. In all patients who underwent arthroscopy, both knee joints where biopsied for ST on the same day. SF was obtained by arthrocentesis. In case of a combined collection of ST and SF, the SF was collected
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