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Heparin Low Molecular Weight

The use of low molecular weight heparin (LMWH) is increasing significantly and it is important that it be used in conjunction with appropriate laboratory monitoring. The chief functional difference between standard (unfractionated) heparin and LMWH is that standard heparin has equivalent inhibitory activity against both factor Xa and thrombin (by binding to and potentiating antithrombin), while LMWH, on account of its shorter polysaccharide chains, preferentially inactivates factor Xa. LMWH has more favorable pharmacokinetics and bioavailability than standard heparin with a considerably longer plasma half-life, and produces a more predictable anticoagulant response. These properties allow LMWH to be administered only once or twice per day.

In view of its predictable anticoagulant effect, low molecular weight heparin (LMWH) can be administered safely and effectively without laboratory monitoring in most cases. Indications do exist, however, in which monitoring may be required. These indications include the following:

  • Renal insufficiency
  • Pregnancy
  • Marked obesity (>180 kg)
  • Low body weight (< 40 kg)
  • Long-term therapeutic doses of LMWH
  • Pediatric patient
  • Unexplained bleeding while on LMWH

The test used for monitoring LMWH treatment is an antifactor Xa assay. The APTT cannot be used since it is affected very little by LMWH. The assay should be ordered as “LMWH assay” and the brand of LMWH noted (eg Lovenox). A 5mL blue-top tube is required. The sample for the assay should be drawn 4 hours after a dose of LMWH. The recommended therapeutic ranges for the LMWH assay are listed below.

Indication

LMWH Therapeutic

Range (U/mL)

Treatment of VTE­

  • Twice daily dose
  • Once daily dose

0.5-1.0

1.0-2.0

Prophylaxis of VTE

0.1-0.2

Pregnant patient

  • Treatment of VTE
  • Prophylaxis of VTE

0.5-1.2

0.2-0.6

­ Venous thromboembolism

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