Loss of response is frequently encountered in patients with inflammatory bowel disease (IBD) treated with antitumor necrosis factor (TNF) agents. Therapeutic drug monitoring (TDM) and antidrug antibody measurement are increasingly used in this setting.To establish a consensus on the use of TDM in the context of loss of response to anti-TNFs, we performed a vote using a Delphi-style process followed by an expert panel discussion among 8 IBD specialists practicing in Switzerland, Europe. Statements were rated on an even Likert-scale ranging from 1 (strong disagreement) to 4 (strong agreement), based on expert opinion and the available literature.The experts agreed on the following statements: (i) loss of response is associated with inadequate drug levels in both Crohn's disease and ulcerative colitis; (ii) best timepoint for measuring drug levels is prior to the next application (= trough levels) with different thresholds for anti-TNF agents (infliximab 5 μg/mL, adalimumab 8 μg/mL, certolizumab pegol 10 μg/mL); (iii) antidrug antibodies are predictive for loss of response; and (iv) antidrug-antibody titers and drug trough levels are key determinants in the treatment algorithm. Data about non-anti-TNF biologics were considered too limited to propose recommendations.A Delphi-style consensus among 8 IBD experts shows that TDM and measurement of antidrug-antibody titers are useful in the context of loss of response to anti-TNF. Optimal cutoff levels depend on the type of anti-TNF. These values are critical in the decision making process. More studies are needed to address the value of such measurements for non-anti-TNF biologics.