Comparison of a fluorogenic anti-FXa assay with a central laboratory chromogenic anti-FXa assay for measuring LMWH activity in patient plasmas.

INTRODUCTION Low molecular weight heparins (LMWHs) are used worldwide for the treatment and prophylaxis of thromboembolic disorders. Routine laboratory tests are not required due to the predictable pharmacokinetics of LMWHs, with the exception of pregnant patients, children, patients with renal failure, morbid obesity, or advanced age. Anti-Factor Xa (anti-FXa) plasma levels are most often employed in the assessment and guidance of accurate dosing in these patient cohorts. MATERIALS AND METHODS A LMWH calibration curve was generated using citrated human pooled plasma spiked with pharmacologically relevant concentrations (0-1.2U/ml) of two low molecular weight heparins; enoxaparin and tinzaparin. Least squares analysis determined the best curve fit for this set of data which returned low sum of squares (SS) values for the log linear fit with an R(2) value of 0.98. 30 patient samples were tested in the fluorogenic assay and concentrations were determined using the log linear regression equation and correlated with a standard chromogenic assay used for heparin monitoring. RESULTS A statistically significant correlation was found between the fluorogenic and the chromogenic anti-FXa assays for 30 patient samples, with a slope of 0.829, offset of 0.258 and an R(2) value of 0.72 (p<0.0001). CONCLUSIONS In the study presented here, a fluorogenic anti-FXa assay was correlated with a standard laboratory chromogenic anti-FXa assay using samples from patients on LMWH therapy. Significant correlations between the values derived by the fluorogenic and chromogenic anti-FXa assays were found for the patient cohort tested in this study.