The relationship between INTEM/HEPTEM coagulation time ratio and heparin plasma concentration in obstetric patients: an exploratory in vitro investigation.

Abstract

Unfractionated heparin continues to be one of the main agents used for thromboprophylaxis in obstetrics, which can complicate the placement of neuraxial anesthetics. In this study, we explored the relationship between a point-of-care coagulation test (thromboelastometry) and plasma heparin concentrations in vitro. We obtained blood from consenting obstetric patients with uncomplicated pregnancies in their third trimester who were not in labour and had a specific hematocrit range. Blood was processed and analyzed. We added increasing amounts of unfractionated heparin to samples from 0 to 0.3 U·mL-1 in 0.05 U·mL-1 increments to simulate increasing doses of unfractionated heparin. We performed INTEM and HEPTEM testing in parallel with activated partial thromboplastin time (aPTT) testing. We created a model of the relationship between heparin concentration and the INTEM/HEPTEM coagulation time (IH CT) ratio using nonlinear regression. A similar model for aPTT was also created. Seventy-seven patients were included in the study. Only one concentration of heparin was added to blood samples of each patient. At a concentration of 0.05 U·mL-1, the IH CT ratio was less than or equal to 1.1 in 9/11 (82%) samples. Activated partial thromboplastin time was not prolonged (> 35 sec) until a concentration of 0.1 U·mL-1 heparin was added. In all samples, the IH CT ratio was prolonged at a concentration ≥ 0.2 U·mL-1 as measured by thromboelastometry; however, at no concentration of heparin was aPTT prolonged in all samples. The point-of-care IH CT ratio may be useful in identifying the presence of little to no heparin activity. Further research is needed to determine if this ratio can predict heparin activity in vivo.