Although the activated partial thromboplastin time (APTT) is a clotting time assay that is useful in screening for hemophilia [8] and the presence of inhibitors [9,10], and for monitoring unfractionated heparin treatment [11], this assay is not suitable for patients treated with emicizumab, because the APTT in patients treated with emicizumab is significantly shortened and is not able to measure FVIII activity in these patients [12]. This evidence concerns the gene F8 and hemophilia.