Monitoring of Nonacog Beta Pegol: One‐Stage Clotting Assay With Kaolin Reagent as a Practical Alternative to Chromogenic Methods

Introduction Accurate monitoring of nonacog beta pegol (N9‐GP) is essential to ensure appropriate treatment and to avoid under‐ or overdosing, which may result in clinical complications. The product's extended half‐life, achieved through molecular modification, poses challenges for activity measurement, particularly when using a one‐stage clotting assay (OSA). Therefore, a chromogenic substrate assay (CSA) is preferred, although it may be less practical for urgent or routine monitoring. Aim To evaluate OSA and CSA for N9‐GP measurement, focusing on the suitability of the OSA STA CK Prest reagent on the STA R MAX analyser. Methods Factor IX activity (FIX:C) of N9‐GP in vitro spiked and post‐infusion samples were measured using various OSA reagents and CSA across different analysers. The in vitro sample results were evaluated as percentage recovery; the OSA STA CK Prest results from both spiked and post‐infusion samples were compared with those obtained by the CSA Biophen FIX. Results Acceptable recoveries for samples spiked at concentrations of 0.04, 0.08, 0.15, 0.61 and 1.22 IU/mL were achieved using CSA Biophen FIX (mean recovery 92% with the CN 3000 analyser and 108% with the STA R MAX analyser) and OSA STA CK Prest (mean recovery 92%, range 86%–111%). STA CK Prest showed FIX:C results comparable to Biophen FIX in both in vitro and post‐infusion samples. Conclusions STA CK Prest and Biophen FIX showed acceptable recoveries for N9‐GP, with FIX:C results in good agreement in both in vitro and post‐infusion samples. This supports the reliability of STA CK Prest for FIX:C measurement.