Apparent heparin resistance from elevated factor VIII during pregnancy.

BACKGROUND: Heparin resistance is the need for more than 35,000 units of heparin per 24 hours to achieve therapeutic activated partial thromboplastin time (APTT) values. Elevated factor VIII can cause apparent heparin resistance by suppressing the APTT result without inhibiting the antithrombotic effect of heparin. CASE: A 41-year-old gravida 2 para 0 presented at 25 weeks of a twin gestation with a deep venous thrombosis that required unusually high doses of heparin, resulting in hematuria. Apparent heparin resistance caused by elevated factor VIII was diagnosed, and the heparin dose was appropriately decreased with anti-Xa heparin monitoring. The deep venous thrombosis and hematuria resolved.
CONCLUSION: Factor VIII rises significantly during pregnancy, and can cause apparent heparin resistance. When this occurs, anti-Xa heparin levels are superior to APTT for monitoring heparin therapy.