![]() Originally defined as the amount of insulin required to cause convulsive hypoglycemia in a fasted 2kg rabbit, 6 standards for potency (IU per mg or equivalent) have changed with improvements in the preparation and stabilization of insulin in solution. ![]() ![]() The history of the insulin Unit has been well reviewed elsewhere, 6, 7 but it is, in short, a history of change. The correct conversion factor between conventional and SI units for human insulin is 1 μIU/mL = 6.00 pmol/L. 2, 5 Indeed, this is the reason the issue came to our attention, as we had reference concentrations that were low in IU/mL, which didn’t make sense. This area of confusion has important clinical and research implications, as insulin concentrations may be ~15% in error if the incorrect conversion factor is used. This issue has been noted several times over the past couple of decades, 1 - 4 seemingly without wide uptake in practice. The issue of real concern is the conversion factor most visible in literature and online conversion tools is based off old and superseded standards, and imprecise measurements of the atomic weight of insulin. The different conversion factors between conventional and SI units can essentially be traced back to differences in standards and purity of insulin preparations. Since its discovery in 1922, the definition of the amount of insulin required to achieve a standard glycemic effect has changed several times. The conventional units (international units, IU or just U) for insulin are based on bioefficacy, rather than the mass/count-based units of the Système International (SI) units.
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