Fish Oil and SI Units

To the Editor.—  Having been in and around the world of medicine for more than 30 years, the decision to implement the use of SI units for publications hardly is surprising to me.¹ Unfortunately, the use of the SI units in certain instances breaks a rule of major scientific importance, ie, data should be reported in the units in which they were measured. Converting milligrams per deciliter to millimoles per liter for reporting is little more than a mathematical exercise when the compound measured has a known and constant molecular weight. It can produce major inaccuracies when the molecular weights either are not known or a calculated figure is used to estimate an average molecular weight. For example, when high-density lipoprotein cholesterol is reported in millimoles per liter, the result given is for cholesterol from a mixture containing at least five major peptide groups, free cholesterol, and many different