The utility of monitoring drug levels in rheumatoid arthritis and spondyloarthritis patients on biological therapy is called into question. The objective was to study relevant clinical questions on the topic, i.e., (1) whether drug levels predict relapse in patients whose biologic was optimized because of remission or low disease activity; (2) whether information about drug levels influences the prognosis of patients with primary or secondary failure to a biological therapy; and (3) whether methotrexate (MTX) influences the association between drug levels and response. Medline, Embase, Cochrane databases were screened, from inception to December 2016 in search for all studies related to the three research questions about. Overall characteristics and outcomes of the studies were collected in a table of evidence and the quality of the studies was assessed with the Newcastle-Ottawa scale or the GRADEpro. Two studies responded the first question, 5 the second, and 7 the third. Studies were small and with limitations, but suggest that measurement drug levels may be useful in patients in remission; that higher drug levels predict a longer relapse-free optimization, and in patients with failure to a biological agent, treatment may need individual adjustment according to the presence of drug levels or antidrug-antibodies. In addition, MTX influences the association between response and drug levels. Monitoring drug levels would allow optimal use of current biological therapies, but more studies and of better quality are needed to draw definitive conclusions.
Keywords: Biologics; Drug levels; Rheumatoid arthritis; Spondyloarthritis; TNF-inhibitors; Therapeutic drug monitoring; Tocilizumab.