Standardisation of Protein S in Plasma: Calibration of the 1st International Standard

Summary

An international collaborative study, involving 16 laboratories, was carried out to calibrate the 1st International Standard Protein S, plasma, for total and free Protein S antigen and for Protein S function. Potency estimates were calculated relative to locally collected fresh normal plasma pools. Estimates of total Protein S antigen showed good agreement between laboratories with inter-laboratory geometric coefficient of variation (gcv%) of 8.8% and a mean value of 0.89 IU/ampoule. Estimates of free Protein S antigen calculated relative to total Protein S antigen in the fresh normal pools were more variable (gcv 26.8%) than estimates calculated relative to free Protein S antigen (gcv 15.3%); the latter comparison was used for calibration with a mean value of 0.89 IU/ampoule. Functional Protein S was estimated using commercial kits which gave an overall mean value of 0.92 IU/ampoule (gcv 14.0%). The 1st International Standard Protein S, plasma, (coded 93/590) was established in October 1995 with a single potency of 0.90 IU/ampoule for all three parameters.

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