Summary

Protein S is an anticoagulant protein that circulates in plasma incomplex with C4b-binding protein (C4BP) or in free form. Deficiencyof protein S increases the risk of venous thrombosis. Measurementof free protein S, as compared to total levels, has been shown to besuperior for prediction of protein S deficiency. We studied the effects ofdifferent handling protocols for an immuno- and a ligand (C4BP)-basedassay for free protein S. When the assay was performed at 37° C, thelevels of free protein S in plasma from protein S deficient patientswere approximately twice those obtained at room temperature. Thereason for this phenomenon was that plasmas from protein S deficientpatients exhibited a time-, temperature-, and dilution-dependent increasein free protein S, which was more pronounced than correspondingdilution of the normal plasma that was used to create the standardcurve. These findings demonstrate the importance of assay procedureand sample handling in assays for free protein S.