Determination of soluble fibrin: a comparison of four different methods

The determination of soluble fibrin (SF) in plasma was compared using four different methods. The SF-ELISA immunologically measures the concentration of desAA- and desAABB-fibrin while the SF-tPA-test is based on activation of plasminogen by tissue plasminogen activator (tPA) in the presence of fibrin; the SF-PS-turbidimetry assay relies on the protamine sulphate (PS) -induced aggregation of fibrin in plasma whereas the SF-erythrocyte-agglutination-test (SF-EAT) detects soluble fibrin by its aggregation with fibrin monomers attached to test erythrocytes. Soluble fibrin was generated in vitro by addition of thrombin or ancrod to plasma. In these experiments the soluble fibrin values of the four methods correlated well with each other and with the fibrinopeptide A release, especially in ancrod-induced fibrinogen turnover (r greater than 0.93). This high correlation is remarkable, considering the fact that the methods are based on different principles. Detection of thrombin-induced soluble fibrin was more sensitive; differences between ancrod and thrombin action were observed as well, probably due to different forms of soluble fibrin. A delayed increase of SF-PS-turbidimetry values in particular during the thrombin action can be attributed to a lack of detectable aggregation of soluble fibrin at low concentrations due to its solubility in plasma. Subsequently, soluble fibrin was measured in samples from patients. The SF-ELISA and SF-tPA-test were highly sensitive and correlated better than the other methods with each other, but all correlations were less satisfactory compared with the in vitro studies. These weaker correlations might be explained by the heterogeneity of soluble fibrin determined by inter- and intraindividually varying concentrations of fibrinogen and its different derivatives in plasma samples from patients. All methods provided reliable results with differences in sensitivity, specificity and practicality. The SF-tPA-test, SF-PS-turbidimetry, and SF-EAT are practical methods for routine use whereas the SF-ELISA is a highly reliable and by far the most sensitive and specific method thus offering new insights into pathogenesis of fibrinaemia and related diseases.