<b><i>Background:</i></b> Up to a third of inflammatory bowel disease) patients show primary nonresponse to antitumor necrosis factor (anti-TNF) biological therapy, and of those who respond, up to 40% develop secondary loss of response (LOR). Therapeutic drug monitoring (TDM) plays a crucial role in assessing patients with LOR to guide therapy by giving more of the drug or switching to a different biological agent. Although reactive TDM is suggested or recommended by the majority of gastroenterology associations, proactive TDM seems to be more controversial. <b><i>Summary:</i></b> In this article, we discuss the updated guidelines on TDM and will also discuss the available data supporting proactive and reactive TDM in patients with Crohn’s disease and those with ulcerative colitis using the different available biological agents. <b><i>Key Messages:</i></b> Therapeutic drug monitoring (TDM) is a valuable tool to aid in inflammatory bowel disease (IBD) therapy optimization. Reactive TDM is widely accepted in IBD patients with suspected loss of response, especially in those receiving antitumor necrosis factor (anti-TNF) agents. Proactive TDM is emerging as a reasonable approach to patients initiated on anti-TNF therapy, specifically infliximab and, to some extent, adalimumab, particularly for patients with severe ulcerative colitis and fistulizing Crohn’s disease. Similarly, TDM may play a role in patients considering de-escalation from combination therapy. To date, proactive TDM is not widely applied to ustekinumab and vedolizumab and more data are required before this becomes part of clinical practice.

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INTRODUCTION The development and course of inflammatory bowel disease (IBD) appears to be influenced by environmental factors. Particularly, smoking has been shown to assume a harmful role in Crohn’s disease (CD) and a protective role in ulcerative colitis (UC). This study aims to examine the effect of smoking on need for surgery in patients with moderate to severe Crohn’s disease (CD) receiving biologic therapy. METHODS Retrospective study of adult patients with CD at a University Medical Center over a 20-year period. RESULTS A total of 251 patients were included (mean age 36.0 ± 15.0; 70.1% males; current, former, and non-smokers: 44.2%,11.6%, and 43.8%, respectively). Mean duration on biologics was 5.0 ± 3.1 years (&gt;2/3 received anti-TNFs, followed by ustekinumab in 25.9%) and a third of patients (29.5%) received more than one biologic. Disease-related surgeries (abdominal, perianal or both) occurred in 97 patients (38.6%): 50 patients had surgeries prior to starting biologics only, 41 had some surgeries after, and 6 had insufficient information. There was no significant difference in surgeries between ever-smokers (current or previous) vs. non-smokers in the overall study group. On logistic regression, the odds of having any CD surgery were higher in patients with longer disease duration (OR = 1.05, 95% CI = 1.01, 1.09) and in those receiving more than one biologic (OR = 2.31, 95% CI = 1.16, 4.59). However, among patients who had surgery prior to biologic therapy, smokers were more likely to have perianal surgery compared to non-smokers (OR = 10.6, 95% CI = 2.0, 57.4; p=0.006). CONCLUSION In biologic-naive CD patients requiring surgery, smoking is an independent predictor of perianal surgery. Smoking, however, is not an independent risk factor for surgery in this cohort after starting biologics. The risk of surgery in those patients is primarily associated with disease duration and the use of more than one biologic.

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