The international system of units (SI) in historical perspective

American medical journals are shifting to selected SI (Système International d'Unités) units for reporting measurements. Limitation of SI units deemed suitable for use in reporting clinical laboratory results stems from recommendations put forth by the International Federation of Clinical Chemistry and the International Union of Pure and Applied Chemistry. Limitations are: 1) the liter as sole recommended unit of volume in concentration measurement; 2) substance concentration (unit mole) favored over mass concentration (submultiples of the kilogram); and 3) discouraging the use of the prefixes hecto-, deca-, deci-, and centi-. Further discussion by the American Medical Association and other organizations is required before consensus in the US medical community can be reached as to extent of and time frame for conversion to SI for reporting clinical laboratory measurements.