Effect of Prothrombin Complex Concentrate versus Frozen Plasma on Divalent Cations and Vitamin K Dependent Factor Function: An Ex Vivo Mechanistic Study.

BACKGROUND

Prothrombin complex concentrate (PCC) has demonstrated hemostatic superiority over frozen plasma (FP) in cardiac surgery, yet the mechanism beyond factor concentration remains unclear.

OBJECTIVES

To test ex vivo whether citrate-mediated depletion of ionized calcium (Ca2+) and magnesium (Mg2+) by FP impairs vitamin K dependent (VKD) factor function, minimized by PCC.

PATIENTS/

METHODS

Heparinized whole blood from healthy volunteers (n=10) was diluted and supplemented with 20% FP or volume-equivalent PCC. Coagulation was assessed by heparinase-modified INTEM (ROTEM) and contact-activated thrombin generation, and validated in cardiopulmonary bypass (CPB) samples (n=6). Factor IX (FIX) and Factor X γ-carboxyglutamate (Gla) domain affinities for Ca2+ and Mg2+ were measured by MicroScale Thermophoresis (MST).

RESULTS

FP reduced Ca2+ from 0.94 ± 0.03 to 0.24 ± 0.01 mmol·L-1 and Mg2+ to undetectable levels. Without exogenous calcium, FP abolished clot formation (clotting time, CT >3600 s) whereas PCC maintained CT (105.4 ± 16.9 s). Low-dose calcium (0.4 mmol·L-1) partially restored CT (1037 ± 575.5 s); adding MgSO4 (0.6 mmol·L-1) improved it to 275.2 ± 56.2 s. Findings held in CPB samples. By MST, the FIX Gla domain bound Mg2+ ∼6-fold more tightly than Ca2+ (Kd 66.1 ± 8.2 vs 381.2 ± 45.3 μM).

CONCLUSIONS

Lowering ionized calcium with FP-derived citrate revealed a functional interaction between magnesium and the Gla domain of VKD factors: at low calcium, restoring magnesium rather than calcium alone recovered thrombin generation and clot formation. PCC preserved the ionic milieu and thrombin generation. Standard coagulation assays potentially underestimate this magnesium dependence because of supraphysiological recalcification.