Comparison of protein S functional and antigenic assays in normal pregnancy

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. (Am J Obstet Gynecol 1996;175:657-60.)