Discordance among assays for monitoring? Anticoagulation during extracorporeal life support

The optimal method for monitoring of anticoagulation in patients on extracorporeal life support (ECLS) is unknown. The objective of this study was to assess the relationship between anti-factor Xa level (anti-Xa; IU/mL) and activated partial thromboplastin time (aPTT; seconds) for monitoring intravenous unfractionated heparin anticoagulation in adult ECLS patients.

Charts of all adult patients cannulated for ECLS from 2015 through 2017 were reviewed and laboratory and heparin infusion data were extracted for analysis. Time matched pairs of anti-Xa and aPTT were considered concordant if both laboratory values were within the same clinically utilized range. A hierarchical logistic regression model was used to determine factors associated with discordance while accounting for patient level effects.

A total of 1016 paired anti-Xa and aPTT values from 65 patients were evaluated. 500 (49.2%) paired samples were discordant with a degree of variability on linear regression (r = 0.315). The aPTT fell into a higher therapeutic range compared to the anti-Xa in 31.6% and lower in 17.3%. Logistic regression demonstrated that discordance was independently associated with time from initiation of ECLS (OR 1.17 per day, p < 0.001), average heparin infusion rate (OR 1.25 per U/kg/hr, p < 0.001), and INR (OR 3.22, p < 0.001).

Nearly half of all aPTT and anti-Xa values were in discordant ranges and discordance is more likely as the time on ECLS and the INR level increase. The use of either assay in isolation to guide heparin anticoagulation may lead to misestimation of the degree of anticoagulation in complex ECLS patients.