Assessment of circulating insulin using liquid chromatography‐mass spectrometry during insulin glargine treatment in type 2 diabetes: Implications for estimating insulin sensitivity and β‐cell function

Abstract Aim To determine the potential impact of the cross‐reactivity of insulin glargine U‐100 and its metabolites on insulin sensitivity and β‐cell measures in people with type 2 diabetes. Materials and Methods Using liquid chromatography–mass spectrometry (LC–MS), we measured concentrations of endogenous insulin, glargine and its two metabolites (M1 and M2) in fasting and oral glucose tolerance test‐stimulated plasma from 19 participants and fasting specimens from another 97 participants 12 months after randomization to receive the insulin glargine. The last dose of glargine was administered before 10:00 PM the night before testing. Insulin was also measured on these specimens using an immunoassay. We used fasting specimens to calculate insulin sensitivity (Homeostatic Model Assessment 2 [HOMA2]‐S%; QUICKI index; PREDIM index) and β‐cell function (HOMA2‐B%). Using specimens following glucose ingestion, we calculated insulin sensitivity (Matsuda ISI[comp] index) and β‐cell response (insulinogenic index [IGI], and total incremental insulin response [iAUC] insulin/glucose). Results In plasma, glargine was metabolized to form the M1 and M2 metabolites that were quantifiable by LC–MS; however, the analogue and its metabolites cross‐reacted by less than 100% in the insulin immunoassay. This incomplete cross‐reactivity resulted in a systematic bias of fasting‐based measures. By contrast, because M1 and M2 did not change following glucose ingestion, a bias was not observed for IGI and iAUC insulin/glucose. Conclusions Despite glargine metabolites being detected in the insulin immunoassay, dynamic insulin responses can be used to assess β‐cell responsiveness. However, given the cross‐reactivity of the glargine metabolites in the insulin immunoassay, fasting‐based measures of insulin sensitivity and β‐cell function are biased.