72 Chapter 4 latter argument an ln transformation of the linear regression is performed. Relative differences were calculated as follows: Since the units of both parameters are different, the values of the relative differences can only be used for the statistical interpretation of the result and cannot be interpreted clinically. Univariable linear regression analysis was used to assess the correlation of relative difference with various FVIII products (using non-exposed samples as a reference), patient characteristics (age, weight, presence of anti-FVIII antibodies) and co-medication. In the multivariable regression analysis, differences associated with exogenous FVIII products were adjusted for anti-FVIII antibodies and vice versa. The application IBM SPSS Statistics for Windows (version 25.0. Armonk, NY: IBM Corp.) was the software used for statistical analysis. RESULTS In the study 87 samples were included, from 70 patients (54 patients with 1 sample, 15 patients with 2 samples, and 1 patient with 3 samples) as patients were allowed to be included in multiple categories. The study population consisted primarily of men (98%) with a mean age of 37 years and a mean body mass index of 23 kg/m2. Of the included samples, 6% had anti-FVIII antibodies ≥0.6 BU/mL and 67% had exogenous FVIII product present (see Table 1 for more patient characteristics). Fifty-eight samples contained one or two FVIII products: mostly one FVIII product, four samples contained two FVIII products and two samples contained an unspecified FVIII product. The correlation between the FVIII activity and FVIII plasma concentration demonstrated an overall R2 of 0.81 (Figure 1). For better interpretation of the lower range, a linear regression figure with ln transformation was made available in Supplemental Figure 1.