Effects of Pre-Analytical Variables on the Anti-FXa Chromogenic Assay when Monitoring Unfractionated Heparin and Low Molecular Weight Heparin Anticoagulation.

Introduction: The purpose of this study was to determine if the anti-FXa (FXa)assay is less affected by pre-analytical variables in monitoring patients on unfractionated heparin (UFH) and low molecular weight heparin (LMWH) than the activated partial thromboplastin time (APTT).

Methods: Twenty-six subjects receiving enoxaparin (LMWH) in varying concentrations were randomly selected and consented for this study. Twenty individuals receiving UFH were also randomly selected and consented. Each study subject had 6 tubes of citrated blood obtained by venipuncture in a one-time blood draw. Two vacutainer tubes were 3.8% sodium citrate, two were 3.2% sodium citrate, and two had an anticoagulant called CTAD. All of the tubes were from Becton-Dickinson, Inc. One tube from each set had a blood to anticoagulant ratio of 9:1. The other tube had an intentional “short-draw” of approximately 6:1 blood to anticoagulant ratio. All six specimens on each subject had an APTT and a FXa assay corresponding to the type of heparin they were receiving performed by a kit from Diagnostica-Stago, Inc., called STA-Rotachrom Heparin Assay. All tests were performed on an STA-R automated coagulation analyzer. Each FXa heparin assay was performed using either a standard UFH or LMWH calibration curve and a HYBRID curve using a specific combination of calibrators. An ANOVA statistical test and descriptive statistics were used to compare each set of results.

Results: The UFH APTT had a mean range of 100.1–127.2 seconds. The 3.8% short draw tube had the highest mean. The ANOVA however was p=0.9845. The UFH FXa mean range with all 6 tubes was 0.32–0.37 IU/mL. The ANOVA for the UFH curve had a p value of 0.9878. With the HYBRID curve the mean range was 0.34–0.39 IU/mL with a p value of 0.9961. When comparing the six UFH results with the six HYBRID curves data the ANOVA p value was 0.9994. The CTAD tubes gave the highest levels of UFH. The LMWH APTT had a mean range of 37.1–41.2 seconds. The highest mean was the CTAD normal draw tube. The LMWH assay curves had a mean range of 0.37–0.46 IU/mL with the CTAD normal draw tube again having the high recovery of heparin. The ANOVA result had a p value of 0.9060. The HYBRID curve had a mean range of 0.42–0.49 IU/mL and again the CTAD normal draw tube displayed the highest amount of heparin. The ANOVA p value was 0.9379. When comparing the 6 LMWH results with the 6 HYBRID results the ANOVA p value was 0.9512.

Discussion: The APTT results were skewed. In one subject the APTT was 48 seconds and another around 114 seconds with the same amount of UFH detected. This makes the so-called therapeutic range suspect. The UFH and LMWH data compared to the HYBRID curves was excellent clinically and statistically. This data demonstrates that the laboratory could use a single calibration curve for monitoring both UFH and LMWH subjects. The intentional short draw tube did not seem to affect the FXa assay. The FXa assay appears to be a better method monitoring heparin subjects than the APTT. The APTT was largely unaffected in subjects on LMWH and is not normally used to monitor individuals on this anticoagulant.