C‐reactive protein‐induced activated partial thromboplastin time prolongation in heparinized samples is attenuated by elevated factor VIII

Abstract Introduction Activated partial thromboplastin time (aPTT) and antifactor Xa (anti‐Xa) activity are used to monitor unfractionated heparin therapy in children on extracorporeal membrane oxygenation (ECMO). Elevated C‐reactive protein (CRP) can prolong aPTT and cause discrepancy between these two assays. We aimed to evaluate CRP effect on aPTT and anti‐Xa assays in the presence of heparin and to determine whether elevated CRP affects laboratory monitoring in pediatric ECMO patients. Materials and methods Citrated normal specimens were spiked with CRP, heparin, and recombinant factor VIII (FVIII) and followed by measurement of aPTT and anti‐Xa activity. Additionally, aPTT, anti‐Xa activity, FVIII, fibrinogen, and CRP were measured in 18 ECMO specimens. Results Elevated CRP prolonged aPTT in normal specimens with or without heparin, but did not affect anti‐Xa assay. In contrast, ECMO specimens showed similar aPTT and anti‐Xa values regardless of CRP level. Elevated CRP in specimens was accompanied by increased fibrinogen and FVIII activity. Additional in vitro experiments confirmed that FVIII spiked simultaneously with CRP attenuated CRP‐induced aPTT prolongation in heparinized specimens. Conclusion In vitro CRP‐induced aPTT prolongation is not observed in pediatric ECMO samples due to concomitant FVIII increase. Discordant changes of CRP and FVIII in plasma could contribute to aPTT/anti‐Xa discrepancies observed during heparin therapy in the pediatric population. The anti‐Xa assay is preferable for heparin monitoring in pediatric ECMO settings.