The Heparin Unit: A Source of Medical Misunderstanding

To the Editor .—Since its discovery in 1916 by McLean, heparin has become an important drug in the armamentarium of the contemporary surgeon. Despite ubiquitous surgical use, many outdated concepts have been perpetrated concerning this drug. An example of such is the employment of the milligram as the unit for selection of heparin dose. The milligram unit for heparin originated in the early ether-aceton extraction techniques of McLean, Howell, and others. The desiccated powder produced by these techniques was weighed and the milligram assigned as the basic unit. Thus, the original unit for heparin was a measurement of weight and not of biologic activity. Following commercial marketing of the drug, the milligram was assigned an arbitrary value of 100 biologic units. J. E. Jorpes, a distinguished contributor to heparin development, succinctly explained this rationale: "One milligram of heparin as used in the clinical literature denotes 100 USP or international units—a