To the Editor.—
Besides problems caused by misreading labels on heparin vials (231:701, 1974), most surgeons in our community still use milligrams in prescribing the dose of heparin to patients. This is probably because the US Pharmacopoeia years ago required not less than 120 USP heparin units/mg for heparin sodium derived from lungs, and not less than 140 USP units/mg for heparin sodium derived from other tissues. So, the conversion factor was accepted roughly as being 100 units/mg. As the purity of heparin sodium continues to improve, the potency may vary from 120 to 180 heparin units/mg or even more.To obtain uniform activity from product to product, heparin sodium injection USP must be prescribed in terms of USP units only.