Background: Two different assays are generally used for the measurement of FVIII activity: a one-stage assay (OSA), based on the activated partial thromboplastin time (aPTT) and a two-stage or chromogenic substrate assay (CSA). Discrepancies between these assays are frequently reported.  At Cliniques Universitaires Saint-Luc Brussels, we follow a population of 250 haemophilic patients. In this respect, we monitor the patients by using both assays.
Methods: We analysed retrospectively 882 laboratory prescriptions for which both tests were executed in diagnosis as well as treatment setting. OSA is realized with the HemosIL Werfen kit, whereas Biophen FVIII:C from HYPHEN BioMed is used for CSA. Clinical discrepancy was defined as a difference in the classification of haemophilia A which is based on the percentage of FVIII: <1% (severe haemophilia), 1-5% (moderated haemophilia), 6-40% (minor haemophilia), >40% (normal value). Two patients receiving emicizumab were excluded.
Results: Of the 865 samples, 12% were discordant. Among these, 67% and 33% presented higher and lower results respectively with OSA in comparison to CSA.
Conclusion: Discrepancies between assays measuring FVIII are frequent, particularly at low values. Even if some authors affirm that CSA improves diagnosis of haemophilia A, in view of our results, we prefer to continue using both tests in our center. Yet, some haemophilia centers only use OSA.  Consensus between haemophilia centers should standardise the management of haemophilia patients. In this study, we emphasize the need to combine both approaches.

Interest of combination of two assays in diagnosis and follow-up of haemophilia A