Seminars in Nephrology Does Not Switch to SI Units

You may not have noticed if, like a sensible person, you do not read editorials, that  there is an attempt to convince (more like coerce) medical writers, and more ominously, laboratory directors to report results in SI units. “SI units” is the abbreviation for Le Systeme internationale d’Unites. This is enough to tell you that we are in deep trouble.

An outgrowth of the metric system, SI has been widely used throughout the world, we are assured (see the reference), but has had little impact in the United States, even though Congress passed the Metric Conversion Act in 1975. I suppose this is the reason that medical science in this country lags so far behind the rest of the world. I also am sure that you remember the Metric Conversion Act of 1975 and that you buy your gasoline by the liter.

The move to SI seems to have been initiated by the American National Metric Council, an organization I am not familiar with, but which seems to be a spiritual kindred of the Universal Esperanto Association. It also seems to inspire great zeal for its cause in its members. Since any change is difficult, there should be a good reason for adopting it, especially one that requires blood gases to be reported in pascals (everyone knows that a pascal is a kg.m-1.s-2). What might it or they be? The ANMC seems to feel that medicine needs a coherent system of units so that there may be effective interchange of information between nations and disciplines. They present no evidence that there is a communication gap resulting from incomprehensible units or that, if such an information gap exists, SI will help. The whole business seems to be presented as a matter of faith. It is ironic that a system advocated by scientists should be put in place in the name of science without its partisans feeling any need for scientific rigor.

Doubtless my failure to grasp the urgency or even the slightest need to switch to SI units represents a personal inability to see the big picture, but the system seems cumbersome, confusing, inconsistent, and unnecessary. SI uses the familiar pascal as the unit of pressure, but magnanimously allows the use of mm Hg for blood pressure, though not for blood gases. I’m sure you can already see how SI is going to simplify your life. Another great concession by the ANMC is their permission of the use of the unit “day” even though it is not an SI unit. Without this dispensation you would have to express urinary protein excretion in grams (I think it’s alright to use grams) per 86,400 seconds.

I hope you realize the seriousness with which this business is approached by those who spend a major portion of their time contemplating the proper application of the SI system. The implications of the system are dissected with talmudic precision. Consider the following:

“Although much debate has centered on the reporting of hemoglobin concentrations with mass concentration (g/L) as well as substance concentration with both the monomer, Hb (Fe), and tetramer, Hb (4Fe), being advocated at different times, the International Committee for Standardization in Hematology indicates greater support for expressing hemoglobin as substance concentration in terms of the monomer. At least for the present, the Medical and Health Coordinating Group recommends continued use of mass measurement of hemoglobin concentrations, so the reporting unit is g/L.”

The quality of the prose aside, one recalls Henry Kissinger’s dictum that academic disputes are so bitter because the stakes are so small.

The SI system that is designed to simplify clinical measurements sometimes becomes so complex that new units have to be invented. Enzyme activity is no longer to be expressed as international units per liter. The SI system’s official unit is mol.s-1.L-1 which it feels, with reason, is difficult to write and difficult for a computer to print. The unit katal (at first I thought these were African cows), abbreviated kat, corresponding to mol/s has been proposed as a special name to report catalytic activity. What we end up with is kat/L. Thus, an abbreviation for a new unit to replace an official unit in place of the old unit is established by Le Systeme internationale d‘Unites. To paraphrase Yossarian, that’s some system that Systeme Internationale.

Don’t be surprised if one day you pick up a lab slip and find that you don’t understand a single value on it. What probably will have happened is that a mole from the ANMC has infiltrated your clinical lab and surreptitiously reprogrammed its computers to SI units. Now you’ll discover that every patient you see requires a clinical pathology consultation to interpret his lab data. Please, how many mm of Hg to a pascal?

What will happen? Not much, I suppose. When European journals switched to SI units many American authors continued to use the units they’d always employed when they sent a paper abroad, with little consequence to its ultimate acceptability. Much to the relief of your secretary, good old American sloth and passive aggressiveness will probably rescue us once more, this time from the Babelian angst of the ANMC. The French, who started this whole thing, have also provided the solution – Vive l‘indifference!


1. Young DS: Implementation of SI units for clinical laboratory data: Style specifications and conversion tables. Ann Intern Med 106:114-129, 1987

Originally published:

Kurtzman NA (ed): Seminars in Nephrology does not switch to SI units. Sem Nephrol:

7:171-172, 1987.