Clinical and epidemiological research immunogenicity, two validated ELISAs (Prevalere Life Sciences Inc., Whitesboro, NY, USA) were used; one determined the pres- ence of antibodies to whole molecule abatacept (including both the CTLA4 and IgG portions) and one detected the presence of antibodies to the CTLA4 ‘tip’ portion alone (CTLA4-T). The primary end point of the study was the immunogenicity rate, expressed as the proportion of patients with an immunogenic response (ELISA) on day 169. In addition to ELISA, an electro- chemiluminescence (ECL) immunoassay (Meso-Scale Discovery, Gaithersburg, Maryland, USA), which is more sensitive than ELISA, was used as a secondary assay and evaluated through- out. ELISA and ECL samples found positive in screening were veriﬁed by a second conﬁrmatory assay and titre determination. Pharmacokinetic assessments were also made in this study and will be presented in a separate report. Safety assessments Adverse events (AEs) and serious adverse events (SAEs) were classiﬁed using the Medical Dictionary for Regulatory Activities (Version 12.1). Injection and infusion reactions were pre-speciﬁed and classiﬁed as SC injection site reactions (AEs at the site of SC injection) and acute infusion AEs (occurring within 1 h of the start of intravenous infusion). Autoimmune events were pre-speciﬁed based on the potential association with biological treatment. Efﬁcacy assessments Efﬁcacy assessments were secondary objectives; DAS28 and the Health Assessment Questionnaire-Disability Index (HAQ-DI) were evaluated at each visit. The proportions of patients achiev- ing low disease activity state (LDAS; DAS28 ≤3.2) and DAS28- deﬁned remission (DAS28 <2.6) were determined. Statistical methods The sample size of 105 patients, allocated 2:1 to receive SC placebo and SC abatacept during period II, was calculated to yield approximately 90% power to detect an absolute differ- ence of 30% in the immunogenicity rate between period II treatment groups at the 5% signiﬁcance level; this assumes an immunogenicity rate of 5% with SC abatacept at the end of period II and an overall discontinuation rate of 5% during period II. Immunogenicity was analysed for patients with serum sam- ples available at the visits of interest (as-observed) in period I (all patients treated during period I), period II (all patients ran- domised and treated in period II) and period III (all patients treated in period II and subsequently treated in period III). Immunogenicity rates are summarised over time using point estimates and within-group 95% CI, with 95% CI for between- group differences calculated at days 169 and 253. A continuity corrected χ2 test was used to compare the immunogenicity rate between groups on day 169 (p value evaluated at the 0.05 sig- niﬁcance level). Safety is reported for each period for all patients who received ≥1 dose of study drug during that period. Efﬁcacy results are pre- sented for patients treated in period II who had data available at the visit of interest (as-observed). Figure 2 Patient disposition. *Most frequent reason for not being treated was no longer meeting entry criteria. †Patients discontinued period II (Withdrawal phase) early due to disease ﬂare and directly entered period III (Reintroduction phase). IV, intravenous; LTE, long-term extension; SC, subcutaneous. 40 Ann Rheum Dis 2012;71:38–44. doi:10.1136/annrheumdis-2011-200344