68 Chapter 4 ABSTRACT Introduction Haemophilia A is a hereditary bleeding disorder caused by a factor VIII (FVIII) deficiency. As biomarker, FVIII activity is used to classify disease severity and to monitor treatment. The one-stage clotting assay (OSA) is performed to measure FVIII activity, but OSA’s limitations may result in misclassification of disease severity or suboptimal monitoring of treatment. Measurement of FVIII plasma concentration with liquid chromatographytandem mass spectrometry (LC-MS/MS) might overcome these challenges. The objective is to investigate the correlation between FVIII activity and concentration, and determinants for differences between the two methods. Methods In this cross-sectional study, all haemophilia A patients receiving standard-of-care were eligible for inclusion. Within the activity categories of <1 IU/dL, 1–5 IU/dL, >5–40 IU/dL, >40–150 IU/dL, and >150–600 IU/dL we randomly selected 15–20 plasma samples, and compared FVIII concentration (LC-MS/MS) to FVIII activity (OSA) with linear regression and Bland-Altman analysis. Potential determinants for differences were analysed with linear regression. Results Inclusion was 87 samples. Bland-Altman analysis demonstrated an overall mean difference of -1% with an SD of 64% between the two methods. Large differences were correlated with the presence of anti-FVIII antibodies (133% [95% CI 81, 185] n = 5) and use of exogenous FVIII products (-37% [95% CI -65,-9] n = 58), e.g., plasma-derived and B-domain modified FVIII products. Conclusion Despite good overall correlation between the two methods, relative differences were large, especially for samples with anti-FVIII antibodies or exogenous FVIII products. These differences may have clinical impact. More research is needed to determine the value of FVIII plasma concentration in comparison with FVIII activity.