Heparin neutralization is essential for accurate measurement of factor VIII activity and inhibitor assays in blood samples drawn from implanted venous access devices

The purpose of this study was to determine the prevalence and effect of hep arin contamination in samples drawn from implanted venous access devices (V ADs, ports) on factor VIII activity and Bethesda inhibitor assay (BU) and t he efficacy of heparinase to neutralize heparin. Plasma samples containing 85, 45, and 2 U/dl factor VIII were spiked in vitro with heparin from 0 to 3 U/ml, Factor VIII activity was assayed with a one-stage clotting assay on paired samples before and after heparinase, 25 mg/mL plasma. Paired patien t samples drawn from VADs were assayed far heparin concentration, factor VI II, and BU before and after heparinase. At all three concentrations of fact or VIII in vitro, the addition of heparin at 0.12 to 0.25 U/mL decreased as sayed factor VIII activity. Heparinase neutralized up to 2 U/mL heparin and resulted in accurate factor VIII determination, Of 105 VAD samples, 47 (45 %) had heparin contamination >0.05 U/ml. Of 47 heparin-contaminated samples , 42 showed decreased factor VIII activity in before/after comparisons. Fal se-positive BU results were detected in 6 of 47 heparin-contaminated sample s. Heparin contamination occurs frequently in samples drawn from VADs and c ould increase costs through excessive factor concentrate use. We recommend that all VAD samples be pretreated with heparinase before the assay of fact or VIII activity or Bethesda inhibitor titers.