Low-molecular-weight heparin

The low-molecular-weight heparins (LMWH) are fractionated heparins as opposed to unfractionated heparin. These drugs are just fragments of the unfractionated heparin. They’re more predictable and have longer half-life than unfractionated heparin and are therefore preferred in most cases.

The LMWHs have a molecular weight of 3 – 4 kDa.

Compounds

• Enoxaparin (Clexane/Klexane®)
• Dalteparin (Fragmin®)
• Fondaparinux

Mechanism of action

LMWHs don’t inhibit all the coagulation factors that unfractionated heparin does; they only inhibit Xa. However that’s more than enough to ensure that LMWHs are at least as safe and effective as unfractionated heparin and are more convenient to use.

Advantages of LMWH vs unfractionated heparin

• No monitoring of aPTT or other parameters is necessary as they’re more predictable
• Patients can be taught to inject themselves subcutaneously at home
• Lower risk of bleeding
• Lower risk of heparin-induced thrombocytopaenia

Disadvantages of LMWH vs unfractionated heparin

• LMWH are excreted renally and therefore cannot be used in severe kidney failure

Monitoring

There is no need to monitor the effect of LMWH, as the anticoagulant effect is much more predictable based on the dose than for UFH and warfarin. One can measure anti-factor Xa activity, but there is no established therapeutic range anyway.