Up to 40% of patients treated with tumor necrosis factor alpha inhibitors (TNFi) do not respond to therapy. Testing drug bioavailability and/or anti-drug antibody (ADAb) levels may justify dosage adjustment or switch to different drugs, enabling a personalized medicine approach. We report a multicenter cross-sectional study on different methods [ELISA and a cell based functional assay (reporter gene assay - RGA)] for drug/ADAb detection, and on the relationship between drug bioavailability and ADAb. 163 patients with rheumatoid arthritis (RA) treated with infliximab (IFX; n = 67), adalimumab (ADL; n = 49) or etanercept (ETA; n = 47) were tested for drug and ADAb levels. Furthermore, we report prospective data from additional 70 patients (59 RA and 11 juvenile idiopathic arthritis - JIA) tested for drug and ADAb levels at baseline (T0) and after 3 (T3) and 6 months (T6) of treatment with ADL or ETA only. IFX-treated patients were not included because of the increasing use of IFX biosimilars. Stringent inclusion criteria were used in order to avoid unwanted variables in both studies; none of the patients used TNFi before the study, and TNFi was used only in combination with methotrexate. Clinical response was defined according to EULAR response criteria. The two assays performed comparably in the comparison study. Accordingly, ELISA was selected for the prospective study because of its feasibility in the clinical setting. The cross-sectional study found ADAb in IFX and ADL treated groups only, that were associated with a decrease in pharmacological drug availability in the blood. Comparable results were found for the ADL-treated group in the prospective study which also showed a relationship between drug/ADAb levels and the loss of clinical response. Altogether our findings support drug and anti-drug Ab monitoring in the real-world clinical setting thus enabling individualized treatment and reducing disability in chronic inflammatory arthritis.