Summary
This collaborative study was designed to assess the performance of commercial methods
for protein S (PS) antigen measurement. Twenty-five different samples were distributed
deep-frozen (24 plasmas) or lyophilized (one plasma) to five laboratories. They were
analyzed blind in each laboratory by the method used locally and by three commercial
methods which included two electroimmunoassays (EIA), Asseraplate-PS (Diagnostica
Stago), Rellplate-S (American Diagnostica) and an ELISA system, Asserachrom-PS (Diagnostica
Stago). 1. Reproducibility. Average between-laboratory coefficients of variation were 15.4%, 17.6% and 25.3%
for Asserachrom-PS, Asseraplate-PS and Rellplate-S. 2. Specificity. Results of all methods showed that PS is underestimated when C4b binding protein
is high. This influence was particularly evident for the ELISA Asserachrom-PS and
disappeared when the antibody-antigen incubation period was prolonged to overnight.
3. Sensitivity. In all laboratories ELISA detected even the lowest PS concentration (4 U/dl), whereas
the two EIAs were less sensitive (lower detection limit 14 U/dl). All methods and
laboratories correctly diagnosed a plasma sample from a PS congenitally deficient
patient. Conclusions. This study shows that better standardization of PS immunoassays is necessary to improve
accuracy and reduce interlaboratory variability before a candidate plasma standard
can be successfully calibrated in an international collaborative study.
