A high variability is observed in the use of rasburicase in our patients. The hematology and pharmacy services have been working jointly to prepare a consensus-based protocol according to which, depending on the patient s risk of developing TLS (tumoral lysis syndrome), standard prophylaxis is administered to low-risk patients (intravenous hydration, alopurinol and urine alcalinization) and rasburicase is administered initially for 1-3 days to patients with high risk of developing TLS.

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Background The assessment of biological drug levels and immunogenicity might be essential in terms of a more effective and rational use of biological therapies and it is dependent upon the establishment of efficient standardized assays or a consensus that could allow a direct comparison of drug levels and anti-drug antibodies (ADA) data with clinical outcome. Objectives To determine whether an enzyme-linked immunosorbent assay (ELISA) performed with two versions of a commercial kit to assess IFX and ADL levels and ADA yield similar results. Methods The diagnostic capability of two ELISA versions [Promonitor® IFX R1 and R2 (V.1), Promonitor® IFX and Anti-IFX (V.2); Promonitor® ADL R1 and R2 (V.1), Promonitor® ADL and Anti-ADL (V.2) kits (Progenika Biopharma, Spain)] was evaluated in patients with RA treated either with infliximab (IFX; n=24) or adalimumab (ADL; n=24) by three different laboratories. The reliability was determined using the Cohen's Kappa coefficient (K), the Pearson's r, the intraclass correlation coefficient (ICC) and the Lin's concordance correlation coefficient (CCC). The Bland-Altman plots of differences between V.1 and V.2 were drawn to compare values of each assay. Results The qualitative discordant results for IFX levels V.1 were 9/24 samples (K= good) and 2/23 V.2 (K= very good), for ADL levels V.1 were 7/24 (K= moderate) and for V.2 were 0/24 (K= very good). For IFX-ADA were 0/24 in V.1 and V.2 (K= excellent), while for ADL-ADA were 1/24 in V.1 and 4/24 in V.2 (K= from very good to good). The quantitative agreement is shown in table 1, we found a good linear association using the Pearson's r, the ICC was questionable to excellent for both versions in IFX and ADL levels. The CCC results were poor in all determinations. Bland-Altman plots for both, IFX and ADL levels demonstrate the differences between both versions (Fig. 1 and 2, respectively). Conclusions We observed a better agreement in the qualitative than in the quantitative results in both, for IFX and ADL levels and for IFX-ADA and ADL-ADA. We have found a good linear association (Pearson's r) but a low agreement (K, ICC and CCC) comparing results for V.1 and V.2. Further interlaboratory investigations are necessary to improve results and determine their possible clinical application. Disclosure of Interest L. Valor: None declared, D. Hernández Flόrez: None declared, I. de la Torre: None declared, F. Llinares: None declared, J. Rosas: None declared, J. Yaque: None declared, E. Naredo Grant/research support: UCB and MSD, Consultant for: Abbvie, Roche Farma, Bristol-Myers Squibb, Pfizer, UCB, General Electric Healthcare, and Esaote, C. Gonzalez: None declared, J. Lόpez-Longo: None declared, I. Monteagudo Consultant for: Abbvie, Roche Farma, Bristol-Myers Squibb, Pfizer, UCB, General Electric Healthcare, MSD and Esaote, M. Montoro: None declared, L. Carreño Perez: None declared DOI 10.1136/annrheumdis-2014-eular.3290

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Background Inpatients with rheumatoid arthritis (RA) the reasons for failure or loss of response to infliximab (IFX) are controversial, so far both IFX serum levels and the presence of antibodies (Ab) anti-IFX have been associated with these events. In this study we evaluated the correlation of serum levels of IFX and anti-IFX Ab in relation to clinical response in patients with RA. Methods We assessed 59serum samples from patients diagnosed with RA treated with IFX (1st line), taken prior to infusion. Patients were classified as treatment response and DAS28 (Disease Activity Score 28 Joints) as sustained response or remission (DAS28 <2.6,> 6 months) or loss of response (no-remission, DAS28> 3.2 with> 1 swollen joint and / or elevated CRP / ESR). The concentrations of anti-IFX and IFX levels were measured using a commercial ELISA kit (Progenika ™) following the manufacturer’s recommendations. The sample size was previously calculated to ensure a sensitivity (0.6) and specificity (0.8), with a confidence interval of 95%, and related to the measurement of clinical activity index DAS28. Statistical analysis to establish the appropriate values for the cutoff in relation to clinical remission was performed using median (percentiles 25-75), U test Mann-Whitney and ROC curves (Receiver Operating Characteristic). Results The medianIFX levels in no-remission RA patients (n = 35) was significantly lower (55 mg / ml, range: 0.0 to 163.5) compared with patients in remission (121 mg / ml; 40.7 to 262.8, p <0.05). We identified the optimal cutoff with ROC analysis for IFX levels as <0.73 mg / ml, which was associated with no-remission, with a sensitivity and specificity of 61% and 59%, respectively. Furthermore, the median titers of anti-IFX in patients in no-remission (0.0 AU / mL, 0.0 to 697.0) was not statistically different than in remission patients (0.0 AU / ml; 0,0-0, 0 UA, p> 0.05). ROC analysis could not be calculated because of the low percentage of positivity for anti-IFX Ab in the selected patient group (n = 10/59, 16.9 %). Conclusions Thecutoff values for IFX serum, rather than anti-IFX Ab determination, may be useful in relation to determine clinical response measured by DAS28 in RA. However, there is a priority in standardizing laboratory techniques (variability inter / intra-assay and inter / intra-laboratory) to validate this information and its possible clinical application. Disclosure of Interest None Declared