Measurement of plasma heparin levels using a fluorometric assay

The performance of a fluorometric heparin assay system was evaluated. The system indirectly measures heparin by its inhibition of thrombin protease action on a synthetic substrate. Our results confirm that within the range of heparin concentrations recommended by the manufacturer, i.e., 0.1-0.8 units/ml, there is an excellent correlation between expected and observed values. Within-day coefficients of variation range from 1% to 2.6%; the between-day coefficient of variation is 6.5. The accuracy of the assay diminishes under 0.1 units/ml. The assay is not influenced by bilirubin, hemolysis, fibrin split products, or lipemia. Results can be generated in less than 5 min. Possible drawbacks are the wide range of acceptability for the supplied heparin standard, and the frequency of pipetting involved.