The International System of Units (SI), commonly known as the metric system, is the international standard for measurement. According to the National Institute of Science and Technology (NIST), the SI rests on a foundation of seven defining constants:  the cesium hyperfine splitting frequency, the speed of light in vacuum, the Planck constant, the elementary charge (i.e., the charge on a proton), the Boltzmann constant, the Avogadro constant, and the luminous efficacy of a specified monochromatic source.

SI and the Regulatory Environment

The Japanese PMDA requires the use of SI units.  For further information, please refer to the PMDA FAQs on Electronic Data Submission.

The US FDA may request US conventional units. The FDA position is explained in the FDA document, "Position on Use of SI Units for Lab Tests".

Regulatory Strategy

CDISC does not provide a specific solution when a sponsor is asked to provide both.  If a Sponsor anticipates an FDA request for conventional units, a common solution is to produce two versions of the affected datasets (e.g., LB): One with standardized results in SI units (i.e., value in LBSTRESU is an SI unit) and one with standardized results in US conventional units (the value in LBSTRESU is a conventional unit).