Factor replacement therapy with factor VIII (FVIII) concentrates is the current standard of care for patients with haemophilia A. Postadministration monitoring of FVIII activity during on-demand or prophylactic treatment is important, for example to guide a suitable dosing regimen. While the use of two-stage chromogenic substrate (CS) assays is increasing, activated partial thromboplastin time (APTT)-based one-stage clotting (OSC) assays are most commonly used to measure FVIII activity in clinical laboratories. Substantial variations in activity measurements have been observed in association with some OSC assay reagents when assessing extended half-life FVIII molecules. Certain silica-based APTT reagents have previously been shown to underestimate FVIII activity with the polyethylene glycol (PEG)-conjugated product turoctocog alfa pegol (N8-GP [ESPEROCT® ]; Novo Nordisk A/S). As a wide range of assay reagents are used in clinical laboratories worldwide, it is essential to establish which can be used to accurately measure activity with modified FVIII concentrates. Here, we describe the approach taken by Novo Nordisk to determine the suitability and accuracy of assays and reagents to measure FVIII activity in samples that contain N8-GP. While accurate activity measurements were possible with all tested CS assays and most of the OSC APTT reagents tested, three APTT reagents that contain silica as a contact activator were found to underestimate N8-GP recovery (APTT-SP, TriniCLOT™, STA® PTT-Automate). The data demonstrate the importance of characterizing the accuracy of each FVIII activity assay. Any limitations should be communicated to treating physicians and the clinical laboratories that test samples containing N8-GP. © 2019 The Authors. Haemophilia published by John Wiley & Sons Ltd.