In the last two decades, the treatment paradigms for Crohn's disease and ulcerative colitis have signifi cantly changed inclusive of a continuously increasing role of biological therapy (anti TNFs). Some patients, however, experience lack or loss of response to biological treatment, and in such cases the management of patients is often empirical. In this review, the authors aim to summarize the available data regarding epidemiology and predictors of loss of response to biological therapy considering the clinical factors and the relationship between serum concentrations, antibodies against biological agents, respectively. Monitoring drug levels and antibodies is expected to play an important role in the management of loss of response (i.e. to confi rm adherence, allow dose adjustment, or provide rationale for switching to another biological agent or to a different class of biological agent) in the coming years. The optimal method of detection and cut-off values are, however, not clear. In clinical practice, meticulous complex assessment of clinical symptoms, confi rmation of active disease by endoscopic or radiological imaging, and excluding complications remain necessary.
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