Abstract
BACKGROUND: Hyperinsulinemia is an important and treatable risk factor for laminitis in horses. OBJECTIVES: Evaluate the Tosoh AIA-360 automated fluorescence enzyme immunoassay for the measurement of serum insulin concentrations in horses, and compare it to five other immunoassays for insulin quantification. ANIMALS: One hundred serum samples from 83 horses were submitted for insulin measurement. METHODS: The Tosoh AIA-360 was assessed against a reference assay (radioactive immunoassay; RIA). Using the same samples, TOS-FEIA, ELISA, and three chemiluminescent immunoassays (CLIA) were assessed for correlation and agreement with RIA. RESULTS: The TOS-FEIA showed excellent correlation with RIA (r(2) = 0.94, p < 0.0001) and good agreement, with a Bland-Altman constant bias (limits of agreement) of -23.8 μIU/mL (-74.6 to 27.0) and Passing-Bablok fit of y = -8.9 + 0.78x. Mean coefficients of variation were 1.8% for intra-assay and 5.7% for inter-assay precision, with mean recovery upon dilution of 104.2%. The assay comparison yielded good or excellent agreement (constant bias, limits of agreement) with RIA in the < 100 μIU/mL cohort for the ELISA (-7.0, -21.4 to 7.4) and the Cobas e CLIA (-31.4, -60.9 to -1.6). Spuriously high results (2 to > 10-fold of RIA result) were obtained in approximately 10% of results from both Immulite 2000 and 2000XPi CLIA analyzers, rendering the agreement poor. CONCLUSIONS AND CLINICAL IMPORTANCE: The TOS-FEIA had acceptable accuracy and precision for clinical use, including at concentrations of insulin < 100 μIU/mL. The ELISA and one CLIA (Cobas e) showed acceptable accuracy, but the Cobas e demonstrated marked bias compared with RIA. Both Immulite CLIA assays exhibited unacceptable accuracy.