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Comparison of protein S functional and antigenic assays in normal pregnancy
J B Lefkowitz1, S H Clarke, L A Barbour
1Department of Pathology, University of Colorado Health Sciences Center, Denver 80262, USA.
Objective:
Our purpose was to determine the effect of pregnancy on the protein S functional assay (clot based), which is used to screen for all subtypes of protein S deficiency states, and to compare its behavior in pregnancy with antigenic assays.
Study Design:
This was a cross-sectional study of 37 normal pregnant women without thromboembolic risks who were tested by both functional and antigenic protein S assays during the first, second, and third trimesters.
Results:
Mean protein S functional levels decline strikingly from the first to the third trimester, all 10 third-trimester patients had functional protein S levels well below the lower limit of the reference range. In contrast, only 3 of 10 third-trimester and none of the second-trimester patients had free protein S antigenic levels below the reference range.
Conclusions:
The protein S functional assay should not be used in pregnancy to screen for the subtypes of protein S deficiency; misdiagnosis and inappropriate treatment could result.