Variability of plasma anti-Xa activities with different lots of enoxaparin

BACKGROUND Previous studies have indicated the variability of anti-Xa activity in different sources of heparin and the variability of different methods used for measuring anti-Xa activity. Manufacturers of low-molecular-weight heparins (LMWHs) determine each lot's anti-Xa activity against the World Health Organization standard, but little information is known about anti-Xa activity variation between lots of LMWH and the impact on reported anti-Xa activity in patient samples. OBJECTIVE To determine the variation of plasma anti-Xa activity in patients receiving enoxaparin when different lots of enoxaparin are used for test calibration. METHODS We obtained 7 lots of enoxaparin containing approximately 10 000 IU/mL and one lot containing approximately 15 000 IU/mL of anti-Xa activity. For each lot, a 2.0 anti-Xa IU/mL dilution was prepared and a calibration curve performed using a chromogenic method. To test the variation in reported results between the different calibration lots, 20 patient samples were tested. Nineteen patients receiving enoxaparin and one patient not receiving enoxaparin (negative control) were tested in a blinded fashion, and the changes in light absorbance recorded. Anti-Xa activity results from tested plasmas were then extrapolated from each enoxaparin lot calibration curve. RESULTS Using Student's paired t-test, there were statistically significant differences between the plasma anti-Xa activities generated from the various enoxaparin lots. In the range of 0.5–1.0 IU/mL of anti-Xa activity, 3 (4.2%) samples had a >0.2 IU/mL difference (maximum difference 0.33 IU/mL) in anti-Xa activity between 2 lots of enoxaparin. For samples that had supratherapeutic anti-Xa activities (1.0–1.5 IU/mL anti-Xa activity), there was a wider variation (>0.2 IU/mL) in anti-Xa activity, which may have resulted in a dosing change. CONCLUSIONS The statistical differences in plasma anti-Xa activities noted between enoxaparin lots are not clinically significant. However, anti-Xa activities in the upper therapeutic and supratherapeutic ranges (>1.0 IU/mL of anti-Xa activity) resulted in a deviation of >0.3 IU/mL in reported anti-Xa activity, which may result in dosing changes.