Immunogenicity is a frequent cause of secondary non-response to tumour necrosis factor (TNF) inhibitors. Drug level measurement and detection of antidrug antibodies have been shown to be cost effective and clinically relevant, and a large number of assays are available for these purposes. It is, however, difficult to compare assays and translate results into clinical meaningful information due to different methodological approaches and a lack of assay standardization. We have analysed infliximab drug levels and antidrug antibodies in 107 patient samples using enzyme-linked immunoassays (ELISA), immunofluorometric assays (IFMA) and reporter-gene assays (RGA). The RGA gave the lowest results for drug levels, whereas the IFMA detected the highest number of antidrug antibody positive sera. Applying individualized therapeutic ranges to each assay resulted in agreement among all three assays in 74% of samples for drug levels and 98% of samples for antidrug antibodies. We found that TNF inhibitor monitoring assays measure on different scales and that the agreement between quantitative results is limited. However, interassay differences can partially be overcome by assay-individualized translations of quantities into categories, which also is necessary for a meaningful clinical application. Our data demonstrate that assays should not be used interchangeably and that direct comparison of quantitative drug levels obtained with different assays should be avoided.