© The Author(s) 2021. The FVIII activity in patients treated with several extended half-life FVIII (EHL-FVIII) agents different when various activated partial thromboplastin time (APTT) reagents were used. The present study examined the difference in clot waveform analysis (CWA) findings and FVIII activity when various APTT reagents and CWA were used. The CWA including FVIII activity was measured using 12 APTT reagents, and the FIX activation based on a small amount of tissue factor assay (sTF/FIX) were examined in reference plasma (RP), EHL-FVIII (Jivi®) and Kovaltry®. The 3 APTT reagents were associated with high variation in the peak time and height in the CWA when analyzing low concentrations of FVIII. The peak time and height could not be measured with one APTT reagent, and there were marked differences in the CWA findings between Jivi® and Kovaltry® among APTT reagents. Several APTT reagents showed a markedly lower FVIII activity with Jivi® than with Kovaltry®. In the FVIII assay, the peak time measured with sTF/FIX did not differ markedly between Jivi® and Kovaltry®; however, the FVIII activity in Jivi® (as measured by the peak height) tended to be higher than in Kovaltry®. The CWA findings for monitoring Jivi® varied for monitoring Jivi® depending on the APTT reagents used, and sTF/FIX assay may be able to measure the EHL-FVIII.