Therapeutic drug monitoring is a strategy utilized to optimize biological therapy. It consists in the drug level measurement before the next dose is due, when the drug concentration reaches the trough level, and includes the detection of potential antidrug antibodies. The monitoring makes it possible to adjust the therapy accordingly - to intensify or change the biologics applied to secure safe and highly effective therapeutics. Therapeutic optimization is based on the nature of therapy failure, which can be caused by pharmacokinetic or pharmacodynamic factors. Pharmacokinetic monitoring can be currently classified as reactive, measuring the drug level and antidrug antibodies during therapy failure, or adverse reactions to the drug applied, and proactive, monitoring patients in clinical remission to retain remission and prevents development of a secondary failure.