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Introduction: Fast and accurate monitoring is crucial in the successful regulation of coagulation therapy. For the treatment of venous thromboembolism, both unfractionated heparin (UFH) and low molecular weight heparins (LMWH) are commonly administered. The chromogenic anti-factor Xa (FXa) assay is currently considered the ‘gold standard’ assay for monitoring LMWH. However different commercial chromogenic methods often differ when tested with the same samples. Fluorogenic anti-FXa assays have the potential to offer greater benefits over chromogenic assays in terms of greater specificity, sensitivity and they are not so influenced by sample opacity or turbidity. Materials and Methods: Commercial plasmas were spiked with pharmacologically relevant concentrations (0–1 U/ml) of UFH, enoxaparin, and tinzaparin. The fluorogenic assay was carried out using previously optimized concentrations of 4 nM FXa and 0.9 μM fluorogenic substrate, in addition to 6.25 μl of 100 mM CaCl2 and 43.75 μl of plasma. The Biophen® and Coamatic chromogenic assays were carried out according to the manufacturer’s instructions. Reaction rates and endpoint values were analyzed and statistical analysis by means of one-way analysis of variance (ANOVA) was performed. Results: The fluorogenic anti-FXa assay was found to have the broadest therapeutic range of 0-1 U/ml with CVs of < 5% for UFH and tinzaparin and CVs < 9% for enoxaparin. Despite their limited measuring range, excellent reproducibility was observed with both chromogenic assays Conclusions: This study indicated that the fluorogenic assay is the most sensitive assay with the broadest dynamic range for monitoring LMWH therapy when compared with standard chromogenic assays.

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 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.