Coagulation laboratory tests

Antithrombin assay

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Last staff update: 11 September 2020

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PubMed Search: Antithrombin assay [title]

Jeremy C. Parsons, M.D.
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Cite this page: Parsons JC. Antithrombin assay. website. Accessed June 16th, 2024.
Definition / general
  • Assays detect antigenic (type I, reduced normal protein, quantitative) or functional (type II, normal amount of defective protein, qualitative) deficiencies of antithrombin (formerly called antithrombin III)
  • Perform functional assay first - if decreased, perform antigenic assay on fresh specimen
  • Family studies may be helpful

  • Functional assays:
    • Are chromogenic, use predominantly amidolytic methods (i.e. through cleavage of an amide bond), employing a synthetic peptide that mimics the natural target of the enzyme
    • Patient plasma is incubated with excess thrombin and heparin
    • Antithrombin neutralizes thrombin, and remaining thrombin is then quantitated with a chromogenic substance
    • The amount detected is inversely proportional to the patients antithrombin
    • Limitations:
      • False levels may be produced if high levels of heparin cofactor II are present; this is eliminated by assays that use inhibition of factor Xa rather than thrombin
      • Newer assays have protease inhibitors to minimize nonspecific substrate cleavage and bovine thrombin
      • Hirudin or argatroban anticoagulation may interfere with thrombin based assays

  • Antigenic assays:
    • Quantification is usually via radial immunodiffusion techniques, although they have coefficients of variation of 40 - 50%
    • Amidolytic assays have CV of only 9 - 14%
    • Also used are latex particles coated with antithrombin antibodies (e.g. LIA)
    • Light absorbance is related to the amount of antithrombin in the specimen
    • Also family studies (first degree relatives)
    • Limitations:
      • Does not detect functional deficiencies by itself
      • If initial antithrombin result is low, should do confirmatory test on repeat specimen
      • Must also exclude acquired causes
  • Acquired causes of low antithrombin levels:
    • Clot formation
    • Surgical procedures
    • Liver disease
    • Nephrotic syndrome
    • DIC
    • Heparin (full dose therapy decreases levels by up to 30%)
    • L - asparaginase therapy
    • Possibly pregnancy or oral contraceptives

  • Acquired causes of high antithrombin levels:
    • Warfarin therapy
  • Specimen / reference ranges:
    • Plasma in sodium citrate tube
    • Levels are lower in newborns; rise to adult levels (112 - 140 mg / liter) by age 6 - 12 months
    • Mildly decreased values (70 - 80%) are unlikely to be associated with thrombosis

  • Indications:
    • Evaluation of individuals with thrombophilia (strong family history or young patient)
    • Also analyze for factor V Leiden and prothrombin G20210A
    • Preferable to not test during the acute phase of a thrombotic event (normal antithrombin value makes antithrombin deficiency unlikely, although cannot interpret mildly abnormal values)
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