Unfractionated heparin has a mean molecular weight of 10–16 kilodaltons (kilodalton = a unit of mass equal to 1000 daltons). Derivatives of heparin have a lower molecular weight (4–5 kilodaltons). Major advantages of low molecular weight heparin (LMWH) over unfractionated heparin:

• Increased bioavailability (the degree to which a drug or other substance is available to the target tissues)

•Prolonged half-life (drug remains active in your body longer) and clearance from the body (allowing once- or twice-daily injections, compared with continuous intravenous administration of unfractionated heparin

• Predictable antithrombotic (anticlotting) response based on the person’s weight and not requiring monitoring

LMWH has a lesser effect on platelets and vessel permeability (a condition of the vessel wall permitting fluid to pass through). In general, LMWH is as effective and safe and causes fewer serious complications compared with unfractionated heparin (Table 1).

Table 1. Treatment Results of Outpatients Given Low Molecular Weight Heparin Compared with Treatment Results of In-Hospital Patients Given Intravenous Unfractionated Heparin

Study Treatment Recurrent DVT Major Bleeding
Study 1
  		Nadroparin
Heparin 		14 of 202 (6.9%)
17 of 198 (8.6%)		 1 of 202 (0.5%)
4 of 198 (2.0%)
Study 2
 		 Enoxaparin
Heparin		 13 of 247 (5.3%)
17 of 253 (6.7%)		 5 of 247 (2.0%)
3 of 253 (1.2%)
Study 3 Reviparin
Heparin		 27 of 510 (5.3%)
24 of 511 (4.9%)		 16 of 510 (3.1%)
12 of 511 (2.3%)

Modified from Gloviczki P, Yao, JST, eds. Hadbook of Venous Disorders, 2nd ed. London: Arnold, 2001, p. 188.

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