The insulin assay currently run in the University of Iowa Hospitals and Clinics core laboratory ("INSULIN ASSAY, PLASMA", LAB632) measures both free and total native human (endogenous) insulin. The measurement of free insulin requires separate steps to remove anti-insulin antibodies, which can occur in patients treated with non-human insulins. An analysis of results in the last year did not find any instance where the free insulin value differed significantly from the total value (estimated incidence less than 1 in 1000 patients). The laboratory also receives inquiries about the cross-reactivity of the insulin assay with the synthetic insulin ‘analogs’ used therapeutically.

To address these issues, the following applies:

1. Effective Wednesday, February 24, 2010, the standard insulin assay run in the core laboratory will become "INSULIN, TOTAL" [LAB7389] and only report total native human insulin. All SmartSets and preference lists that contain “INSULIN ASSAY, PLASMA” will be updated with this test.

2. If determination of free and total insulin is needed, that can be ordered as "INSULIN, FREE AND TOTAL". This may be clinically indicated in a patient who has been treated in the past with bovine or pork insulins, or if the total insulin values seem discordant with clinical history.

3. The "INSULIN, TOTAL" assay does NOT cross-react with the analog insulins lispro (Humalog), aspart (NovoLog), and glargine (Lantus). This assay is designed to measure native human insulin only. A comment to this effect will be appended to insulin assay results.

4. If measurement of analog insulins is needed clinically (e.g., suspected surreptitious use of insulin injections in work-up of hypoglycemia), the mail-out test "INSULIN, RANDOM (MAILOUT)" (LAB3428) would be appropriate. This assay cross-reacts approximately equally with both native human and therapeutic insulins, including the analogs lispro, aspart, and glargine. There is currently no assay available for clinical testing that only measures synthetic insulins and not native human insulin.

Questions should be directed to Matthew Krasowski, MD, PhD, medical director of the Clinical Chemistry Laboratory (384-9380, matthew-krasowski@uiowa.edu).