Coagulation laboratory tests

Protein S assays

Topic Completed: 1 November 2012

Minor changes: 23 September 2020

Copyright: 2002-2022,, Inc.

PubMed Search: Protein S assays [title]

Jeremy C. Parsons, M.D.
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Cite this page: Parsons JC. Protein S assays. website. Accessed January 17th, 2022.
Definition / general
  • Deficiencies are either quantitative (type I - reduced normal protein) or qualitative (type II - normal amount of defective protein)
  • Assays are either functional (measure protein activity) or antigenic (immunoassays that measure quantity, not function)
  • Gold standard to measure free protein S or APC cofactor activity of protein S is considered the polyclonal ELISA with or without polyethylene glycol precipitation, although this procedure has poor reproducibility
  • Perform functional assay first (detects all types of deficiencies)
  • Functional assays are clot-based, cannot be performed in patients taking hirudin or argatroban
  • Antigenic assays measure free protein S (functionally active form) or total (bound plus free) protein S - usually 60% of protein S is bound to C4b-binding protein
  • Free protein S levels in protein S deficient patients are very sensitive to timing, temperature and dilutional conditions of assays compared to normal individuals
  • More common than hereditary deficiencies - clot formation, surgery, liver disease, warfarin (should be discontinued at least 10 days prior to testing), nephrotic syndrome, DIC, L-asparaginase therapy, any stimulus to acute phase response (increases C4b binding protein, decreases free protein S), newborns (12-60% of adult levels, rise to adult levels by 6 months), women (lower than men before menopause, while taking oral contraceptives, during pregnancy or with hormone replacement therapy), vitamin K antagonist drugs, vitamin K deficiency, elevated factor VIII levels (> 200%) in PTT based functional assays or thrombosis; also nephrotic syndrome, varicella infection and HIV infection
  • Classification of deficiencies: all have low functional protein S; I - also low free and total protein S; II / IIb - also normal free and total protein S; III / IIa - low free, but normal total protein S
  • Methodology:
    • Clot based protein S method is based on the addition of activated protein C, which in the presence of protein S, accelerates the inhibition of thrombin-activated factors VIII and V
    • The prolongation of clotting time is proportional to the amount of factor S activity
    • Interference may occur with elevated factor VIII (acute phase reactions or otherwise) (Thromb Res 1995;77:375)
    • Values falsely increased by bivalirudin, lepirudin, argatroban and fondaparinux (Arch Pathol Lab Med 2004;128:1142), lupus anticoagulants

  • Reference ranges (nmol/liter)
    • Each lab should establish its own, values in acute phase plasma are higher:
    • Total protein S: - 65% of value in pooled normal human plasma (289 - 397)
    • Free protein S: 71 - 115
    • C4 binding protein beta+: 228 - 310
    • Total C4 binding protein: 257 - 423
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