Slide 3:

Physical properties are given for the four LMWHs approved for use in the United States and/or Canada. It should be noted that the plasma half-life of UH is one hr. or less. Because of their smaller size, LMWHs in vitro inhibit clotting factor Xa more effectively than IIa (thrombin). This relationship is often expressed as a Xa/IIa ratio. LMWHs typically inhibit Xa 2-4 times more readily than IIa. The small differences between Xa/IIa ratios for the LMWHs do not appear to be clinically important.