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Background: High levels of infliximab (IFX) directed antibodies (IFX-Ab) may result in significant reduction in IFX concentration and loss of drug efficacy., Objectives: To assess the input of measuring serum IFX levels and levels of IFX-Ab in the management of rheumatic diseases., Methods: Serum levels of IFX and anti-IFX-Ab were measured by ELISA (IFX-Abs were also identified by anti-human lambda chain Ab) and correlated to patients (responders and nonresponders) disease activity scores., Results: A total of 144 tests for IFX were performed in 91 patients (mean age 50.2 years and disease duration 9.9 years). Among responders (57 patients) levels (mean, median) of IFX were significantly higher than in non-responders (34 patients) (4.2 mcg/ml (2.3) versus 1.1 mcg/ml (0.45)); levels of IFX-Ab in responders were significantly lower than in non-responders (4.59 mcg/ml (1.0) versus 13.1 (6.1)). High IFX-Ab levels predicted IFX discontinuation in 8.8% of responders and 55.9% among non-responders. In non-responders with low IFX levels and low IFX-Ab, the shortening of re-treatment intervals lead to significant improvement. In about 28% of patients, results of blood tests influenced treatment decisions., Conclusions: Assessment of immunogenicity of anti-TNF monoclonal antibodies proved useful information for guiding the therapy in rheumatic diseases with suboptimal clinical response. Patients with low IFX levels and low levels of IFXAb may benefit from increasing the drug dose or decreasing of re-treatment intervals. In patients with negligible serum levels of IFX and high levels of IFX-Ab, the therapy should be switched to another biological agent, probably with a different mechanism of action.