Evaluation of a chromogenic assay to measure the factor Xa inhibitory activity of unfractionated heparin in canine plasma

BACKGROUND: Unfractionated heparin (UFH) has a complex pharmacologic profile that necessitates patient monitoring to prevent inadequate anticoagulation or overdosage and hemorrhage. Factor Xa inhibitory assays (to measure anti-Xa activity) are used to adjust UFH dosage and define safe and effective regimens for specific thrombotic disorders in humans. OBJECTIVE: In this study, the accuracy, linearity, and clinical utility of a chromogenic assay were assessed for monitoring UFH anti-Xa activity in canine plasma samples. METHODS: A commercial assay (Rotachrom Heparin, Diagnostica Stago, Parsippany, NJ, USA) was used to measure anti-Xa activity in canine plasma samples spiked with different concentrations of UFH. Background absorbance and assay linearity were compared for canine and human plasmas. Percentage recovery of UFH anti-Xa activity and intra- and interassay imprecisions were investigated by multiple measurements of canine plasma to which known amounts of UFH were added. The spiked plasma samples also were used to determine the heparin sensitivity of an activated partial thromboplastin time (aPTT) test. RESULTS: Canine plasma samples were assayed at a higher dilution than were human plasma samples (3:8 versus 4:8) to eliminate higher background anti-Xa activity in canine plasma. Using this modification, the recovery of anti-Xa activity in canine plasma was linear (R2 > .9) at concentrations of 0 - 0.75 U/mL UFH. Intra- and interassay imprecisions for plasma samples containing 0.5 U/mL UFH were <10%, whereas samples containing 0.25 U/mL UFH had imprecisions of 13% and 24%, respectively. The anti-Xa activity range of 0.5 - 0.75 U/mL caused prolongation of aPTTs to 1.5 - 2.5 times the assay mean. CONCLUSION: Plasma anti-Xa activity of dogs treated with UFH can be accurately monitored using this commercially available chromogenic assay.