Evaluation of Therapeutic Drug Monitoring in the Clinical Management of Patients with Rheumatic Diseases: Data from a Retrospective Single-Center Cohort Study

Purpose: Treatment of rheumatic diseases with tumor necrosis factor inhibitors leads to improved clinical outcomes. Therapeutic drug monitoring (TDM) may assist in guiding clinical decisions. This study investigates the impact of TDM on clinical outcome, decisionmaking and biologics cost expenditure. Patients and Methods: In a retrospective observational study of 306 patients with rheumatic diseases treated with four different tumor necrosis factor inhibitors, drug levels and antidrug antibodies were measured over a period of one year. Primary outcomes were the clinicians' response to each TDM result and the clinical outcome two years after TDM initiation. Outcomes were compared between the 111 TDM-guided patients and the 195 empirically guided patients. Results: Treatment change occurred in 55% of the patients in the TDM group, but in only 38% in the empirically guided group. In the TDM group, 89 (79.5%) patients were in remission or had low disease activity after two years follow-up compared to 128 (65.6%) patients in the empirical group. The average cost of biologics per patient per year was lower in the TDM group than in the empirical group for patients receiving infliximab, adalimumab or etanercept at baseline but not for golimumab. Conclusion: TDM-guided decision-making is useful in rheumatic patients receiving TNFi and may optimize therapeutic decisions, leading to a better control of disease activity. Proactive TDM may support decisions on dose tapering, resulting in lower drug consumption and biologics cost expenditure.