Feline insulin levels are not reliably measurable. Prior methods for measuring levels were assays developed for measuring human insulin. Feline insulin differs from human insulin by 4 amino acids; therefore, an assay that can distinguish between human and feline insulin may be useful. Insulin was measured in 36 healthy, fasted cats and in 22 diabetic cats to investigate the ability of a commercially available ELISA to measure feline insulin and evaluate its stability. The authors were able to support the use of the feline insulin ELISA. Serum insulin was relatively unstable after 24 hours at 20°C, but samples kept at 2°C–8°C were stable for 4 days. Recovery after spiking and dilution of samples was deemed acceptable, although insulin concentrations in the study cats varied. Position of the samples on the microplate significantly affected results and should be assessed in future ELISA validation studies.

Source
Validation of an enzyme-linked immunosorbent assay for measurement of feline serum insulin. Strage EM, Holst BS, Nilsson G, et al. VET CLIN PATHOL 41:518-528, 2012.