16 Chapter 1 The standard method for measuring the concentration of emicizumab in clinical practice is based on a calibrated, modified clotting assay. This assay is not commonly adopted in clinical laboratories, and moreover, assay interference by high concentrations of FVIII concentrates or anti-emicizumab antibodies has been reported . Figure 2. Monitoring purposes of factor VIII and emicizumab. Mass spectrometry: A novel technique in haemophilia A In the last decennium, liquid chromatography (LC) coupled with tandem mass spectrometry (MS/MS) has rapidly become the state-of-the-art technique for TDM in the laboratory landscape. Advantages of this technique over other techniques, such as clotting assays or enzyme-linked immunosorbent assays, include high sensitivity, speed, resolution, accuracy and reproducibility. In addition to these analytical benefits, LC-MS/ MS methods offer the potential of multiplexing, leading to more efficient measurement runs. Especially advantageous is the low sampling volume, which offers minimal blood drawing for children and the potential of dried-blood-spot measurements. Additionally, LC-MS/MS analysis will likely continue to grow as one of the leading techniques for the quantification of proteins (i.e., proteomics) in a clinical setting.