IR in patients with positive results of antibodies binding to FVIII, PEG-FVIII, or PEG. IR was calculated as an increase of FVIII level in IU/dL after infusion from the value before infusion divided by the dose per body mass in IU/kg. Box-and-whiskers plots of IR before (“before positive”), during (“while positive”), and after (“after positive”) any positive results for binding antibodies were drawn with the boxes featuring medians and quartiles whereas the whiskers extended to 1.5 times the interquartile range from the boxes. One patient each in the “while positive” group and in the “after positive” group had an apparent IR close to 0. FVIII level by the 1-stage clotting assay changed from 25.4 IU/dL to 24.4 IU/dL in the former, and from 1.3 IU/dL to 1.0 IU/dL in the latter. Both patients had IRs in the expected range both before and after reporting an apparent IR close to 0; therefore, it is likely that these values close to 0 represent preanalytical mistakes, such as errors in sample labeling or sample preparation.