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Antifactor Xa for LMWH Monitoring

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, LMWH can be administered safely and effectively without laboratory monitoring, however indications do exist in which monitoring may be required. These indications include renal function impairment (since the kidney is the primary clearance route of the drug), pregnancy (due to altered pharmacokinetics of the drug), obesity (>80kg, due to uncertainty regarding the weight-based dose), and unexplained bleeding while on LMWH. I

If laboratory monitoring is required, an anti-factor Xa assay is recommended, as LMWH has very little effect on the APTT. The LMWH assay is very similar to the “heparin assay” used for standard heparin, except that LMWH is used as a calibrator instead of unfractionated heparin.

Therapeutic range is 0.5 – 1.0 U/mL

Specimen requirement is one light blue top (sodium citrate) tube of blood drawn 4-6 hours after a dose of LMWH. The sample should be drawn after a 5 mL plain red top discard tube to avoid tissue fluid contamination. It should be delivered to the laboratory within two hours. The assay should be ordered as “LMWH assay”.

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