Insulin levels in humans were measured by a new assay, the isotope dilution assay (IDA), based on stable isotope dilution mass spectrometry. A known amount of a deuterated analog of insulin was used as an internal standard and added to the serum samples before sample processing. After isolation by immunoaffinity chromatography and solid phase extraction, followed by a purification step on reversed-phase microbore high-performance liquid chromatography (HPLC), the insulincontaining fraction was analyzed by mass spectrometry. The relative intensity of the signals due to insulin and its deuterated analog in the mass spectrum was used to determine the concentration of insulin in the sample. Using serum samples of 0.5–2.0 ml, we were able to measure insulin levels in the range of 3–1700 pmol/l in several clinical samples from type II diabetic patients. The basal level of endogenous insulin was also determined in two normal subjects and found to be∼20 pmol/l. Insulin secretion was followed after the ingestion of 75 g glucose in one healthy volunteer. Finally, the determination of the insulin level of one hemolyzed post-mortem blood sample, for which immunoassays gave inconsistent results, was performed to help forensic investigations. Our results showed a good correlation with standard immunoassay data, except in six samples where much lower values were obtained by our stable isotope dilution assay, suggesting an overestimation of insulin levels by immunoassay in some cases. As it is not subject to immunological interferences by insulinrelated compounds, this new assay has a major clinical advantage in that it avoids confusions related to hyperinsulinemia.

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