Letter: detection of infliximab levels and anti-infliximab antibodies - comparison of three different assays; authors' reply

*Department of Gastroenterology and Hepatology, University Medical Center Groningen, University of Groningen, Groningen, the Netherlands. Laboratory for Pharmaceutical Biology, KU Leuven, Leuven, Belgium. Department of Hospital and Clinical Pharmacy, University Medical Center Groningen, University of Groningen, Groningen, the Netherlands. Division of Gastroenterology, UZ Leuven, Leuven, Belgium. Laboratory for Monoclonal Therapeutics, Sanquin Diagnostics, Amsterdam, the Netherlands. **Department of Immunopathology, Sanquin Research and Landsteiner Laboratory Academic Medical Centre, Amsterdam, the Netherlands. E-mail: d.buurman@umcg.nl Both first authors contributed equally. Both last authors contributed equally.