Low molecular weight heparin: a possible cause for higher protein S activity than free protein S concentration

Different assays for the assessment of protein S (PS) functional activity are commercially available. We were able to show that, considering the influence of factors known in respect of PS, good agreement can be reached between the results of the determination of free PS as obtained using an immunoassay with monoclonal antibodies and the determination of PS activity as obtained using a test based on activated factor X (factor Xa). However, values of PS activity higher than free PS concentration were obtained in plasma samples taken from patients undergoing therapy with low molecular weight (LMW) heparin. An in vitro incubation of plasma samples with LMW heparin in varying concentrations led, in every case, to an increase of clotting times and thus to an increase of PS activity. In all investigations, the ratios of clotting time with heparin to that without heparin were higher in plasma samples containing PS than in PS-deficient plasma. This result was independent of the use of commercially deficient plasma or the blocking of PS in reference plasma by addition of polyclonal PS antibodies. Obviously, heparin blockers in commercially available assays only neutralize the effect of conventional heparin, and the prolongation of the clotting time is mainly caused by the inhibition of factor Xa by LMW heparin. The reason for the stronger effect in plasma containing PS than in the same plasma after the blocking of PS with polyclonal antibodies as well as in PS-deficient plasma is unclear. Due to the unrecognizable influence of LMW heparin on global clotting assays, the assessment of PS activity values without clear documentation of the application of LMW heparin can lead to improper diagnoses.