A recent publication questioned the integrity of insulin purchased from U.S. retail pharmacies. We sought to independently validate the method used, isotope dilution solid-phase extraction (SPE) liquid chromatography mass spectrometry (LC-MS), and expand analysis to two U.S. Pharmacopeia (USP) methods (high-performance LC with ultraviolet detection and LC-MS).Each method was used to evaluate nine insulin formulations, purchased at four pharmacies, within five geographic locations in the U.S.All human and analog insulins measured by the USP methods (n = 174) contained the expected quantity of active insulin (100 ± 5 units/mL). When using isotope dilution SPE-LC-MS, units-per-milliliter values were well below product labeling due to unequal recovery of the internal standard compared with target insulin.Insulin purchased from U.S. pharmacies is consistent with product labeling.

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