Biological drugs are commonly used to treat chronic inflammatory diseases like rheumatoid arthritis and inflammatory bowel diseases. The most common biological drugs are infliximab, adalimumab, and etanercept. Monitoring drug levels and antibodies against the drugs is important because immunogenicity can lead to loss of treatment efficiency. Clinical studies have shown correlations between low drug levels, high antibody levels, and poor patient responses. Monitoring biological drugs allows for optimization of treatment regimens.