Heparins: Heparin and Low-Molecular-Weight Heparin

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The page below is a sample from the LabCE course Antiplatelet and Anticoagulant Pharmacology for the Laboratory Professional. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Heparins: Heparin and Low-Molecular-Weight Heparin

  • Heparin is a large molecule that is very acidic. Enoxaparin is an example of a low-molecular-weight (LMW) heparin with the same mechanism of action and use as heparin but is smaller in molecular weight.
  • LMW heparin is longer acting when compared to heparin and can be dosed less frequently.
  • Heparins as a class cannot be absorbed by the gastrointestinal system, so they are administered intravenously or subcutaneously.
  • Heparins are safe in pregnancy.
  • Mechanism of action: Heparin and LMW heparin functions are dependent on antithrombin III (ATIII), a component of the hemostasis system discussed earlier. ATIII normally inhibits factor IIa (thrombin) and factor Xa, thus causing anticoagulation. Heparins enhance the activity of ATIII, leading to very fast anticoagulation. Heparin can also act on other factors in the intrinsic pathway.
  • Use: Heparins are used when the patient needs anticoagulation quickly (e.g., deep vein thrombosis, myocardial infarction). They are also used in the hospital setting as deep vein thrombosis prophylaxis, as patients are often immobile and subjected to increased risk of deep vein thrombosis.
  • Laboratory measurement: The aPTT test (measured in seconds) is used to monitor heparin and will be prolonged after the administration of heparin. An aPTT result of 1.5x to 2.5x the reference range is the goal. NOTE: LMW heparin is NOT monitored with the aPTT.
  • Toxicity: Anticoagulants cause excessive bleeding if not dosed properly. Protamine is a basic molecule that can be used to neutralize the acidic heparin.