The ideal method for monitoring unfractionated heparin (UH) therapy is unknown. The activated partial thromboplastin time (aPTT) does not measure a heparin level, but rather its ex-vivo “physiologic” response. The anti-activated factor X (Xa) assay measures heparin levels indirectly, and is considered the “gold standard” to which laboratories reference their aPTT therapeutic ranges. Critically ill patients have alterations in the level of multiple proteins theorized to lower the accuracy of the aPTT. Anti-Xa assays are recommended for monitoring of UH therapy in several clinical situations, but there is little information on its use in the intensive care unit (ICU). We sought to determine the concordance between the aPTT and an anti-Xa assay in ICU patients receiving UH.
Prospective evaluation of forty-one consecutive ICU patients receiving continuous UH infusions. Blood samples drawn to monitor UH therapy had both an aPTT and an anti-Xa level measured. Our laboratory’s therapeutic aPTT during the time of the study was defined as 56-94 seconds. The therapeutic anti-Xa level was defined as 0.3-0.6 U/mL.
A total of 273 simultaneous aPTTs and anti-Xa levels were analyzed. The aPTT and anti-Xa level were in agreement (i.e. both in therapeutic or both out of therapeutic range) only 50.9%. The measure of agreement beyond the amount expected by chance alone, determined by the Kappa statistical test, was poor (k=0.24).
The concordance of the aPTT and an anti-Xa assay in determining if critically ill patients receiving UH are in the therapeutic range is extremely poor.
Although, anti-Xa assays are considered the gold standard, the paucity of clinical outcomes data using the anti-Xa for monitoring of UH therapy in critically ill patients, combined with the poor concordance with the aPTT (for which there are clinical outcomes data) prevents us from recommending routine anti-Xa monitoring of critically ill patients receiving UH therapy at this time.
J.A. Rettmann, None.