title: Normal functional protein S activity does not exclude protein S deficiency.
abstract: Protein S (PS) deficiency appears to increase the risk of venous thrombosis. PS deficiency is classified into three phenotypes using antigenic levels and functional activity. By definition, all three phenotypes of PS deficiency should result in low activated protein C cofactor activity. We compared the results of functional PS activity testing to free antigenic PS testing in order to determine if a normal functional PS activity assay result could eliminate the need for free antigenic PS testing. The sensitivity of the functional assay is 45.5% (95% confidence interval, CI, 36-55%), specificity 95.3% (95% CI 93-97%), negative predictive value 88.6% (95% CI 86-91%) with a positive predictive value of 68.5% (95% CI 57-79%). In conclusion, a normal functional PS activity result does not exclude free antigenic PS deficiency. Functional PS activity testing should not be used as a screening test to eliminate free antigenic PS testing for the laboratory diagnosis of PS deficiency.
authors.affiliation: Department of Medicine, University of Ottawa, Ottawa, ON, Canada. mrodger@ohri.ca
authors.forename: Marc A
authors.initials: MA
authors.lastname: Rodger
authors.forename: Marc
authors.initials: M
authors.lastname: Carrier
authors.forename: Muriel
authors.initials: M
authors.lastname: Gervais
authors.forename: Gail
authors.initials: G
authors.lastname: Rock
mesh_terms: D020016:Activated Protein C Resistance / Q000175:diagnosis; D003951:Diagnostic Errors; D005189:False Positive Reactions; D006801:Humans; D010641:Phenotype; D011237:Predictive Value of Tests; D011486:Protein C / Q000378:metabolism; D017293:Protein S / Q000032:analysis*; D018455:Protein S Deficiency / Q000175:diagnosis*; D012680:Sensitivity and Specificity
chemical_list: D011486:Protein C; D017293:Protein S