In patients with rheumatoid arthritis (RA), the development of antibodies to infliximab (ATI) is associated with poor clinical response.1 ,2–7 Nevertheless, there is no plausible explanation for why not all patients with ATI experience high disease activity. To investigate whether the timing of ATI appearance and/or drug disappearance is correlated with clinical activity, we measured the infliximab (Ifx) and ATI levels in patients 4 weeks after infusion (n+4).

Eleven ATI-positive patients with RA receiving Ifx every 8 weeks were included. Disease activity was assessed by DAS28 and European League Against Rheumatism (EULAR) criteria at baseline, and during weeks n and n+8. The serum drug and ATI levels were determined using a capture and bridging ELISA, as previously described,1 ,6 ,8–10 at weeks n, n+4 and n+8.

Patients were divided according to the findings from week n+4: Group 1 (ATI-positive=without Ifx and ATI-positive, ATI-negative=with or without Ifx and ATI-negative); Group 2 (with Ifx=ATI-negative and with Ifx, without Ifx=ATI-positive or ATI-negative and without Ifx). Table 1 shows the patients’ demographic and clinical characteristics. At week n+8, four patients were moderate responders and …