Evaluation of fasting plasma insulin and proxy measurements to assess insulin sensitivity in horses

Evaluation of an Automated Fluorescence Enzyme Immunoassay for Quantification of Equine Insulin and Comparison to Five Other Immunoassays

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 (r2 = 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.

Obesity and gestation in Criollo mares: endocrine and metabolic profiles

The aim of this study was to determine the effect of body condition score (BCS) on metabolic and endocrine parameters in pregnant Criollo mares (n=41), which were categorized according to their BCS as obese (7 to 9 BCS, n=26) or normal (5 to 7, n=15). Blood samples were taken during gestation in 3 periods: between 3.5 and 5 months (I), 8 and 9 months (II) and in the last month of gestation (III). The data was analyzed in the statistical model by mixed procedures, including BCS, gestational period and their interaction as fixed effects. BCS was only different in period I, as normal mares increased their BCS in the later periods. Leptin concentrations were greater in obese mares when compared to non-obese mares during all sampling periods (P<0.01), while glucose concentrations were also greater in the former group (P<0.01) but only during the first sampling period. Insulin concentrations and homeostatic model assessment for insulin resistance (HOMA-IR) index were greater also in obese mares in periods I and III (P<0.05). Adiponectin concentrations in period I were lower in obese mares (P<0.05). Cholesterol concentrations increased during gestation, and obese mares tended to have greater concentrations than nonobese mares (P<0.1). Triglyceride concentrations were not affected by group or gestational period. This study revealed adaptations in carbohydrate and lipid metabolism during gestation in mares. Several parameters are dependent on the degree of body fat reserves, which are reflected in the concentrations of biomarkers such as leptin and adiponectin. Insulin concentration in obese mares was higher than non-obese mares at the end of gestation, a similar profile was observed for HOMA-IR although cutoff values are yet still to be validated.

Comparison of one novel and four established diagnostic tests for insulin dysregulation in ponies

Several tests have been advocated for diagnosis of insulin dysregulation (ID). Tests using simple sugars may not reflect the response to naturally ingested carbohydrates. This study aimed to evaluate agreement between the oral glucose test (OGT), the oral sugar test (OST), a novel oral test using a proprietary cereal (WEET), the IV combined glucose-insulin tolerance test (CGIT) and fasted basal insulin (FI) for diagnosis of ID. Each of the five tests above was performed on a group of six normal and six insulin dysregulated mixed-breed ponies in a randomised crossover study. Area under the curve (AUC) and maximum concentration of insulin from OGT, OST and WEET showed strong to very strong bivariate correlations (r = 0.85-0.94, and r = 0.87-0.92, respectively; P ≤ 0.001) and were significantly different between tests (associated with dose of carbohydrate) and between CGIT-positive and -negative ponies. Dichotomous results showed substantial agreement between OST and both WEET (κ = 0.65; P = 0.02) and OGT (κ = 0.67; P = 0.01) and between CGIT and both OST (κ = 0.63; P = 0.03) and OGT (κ = 0.67; P = 0.01), and no agreement between FI, which had low sensitivity, and all other tests (κ = 0.15 - 0.31; P > 0. 05). Palatability of WEET was variable, resulting in one pony being excluded for analysis of WEET data. Further work on development of an oral test using a more palatable feedstuff and appropriate cut-offs or diagnostic thresholds for tests of ID is warranted.

Development of a Web App to Convert Blood Insulin Concentrations among Various Immunoassays Used in Horses

The measurement of the blood insulin concentration, and comparison to cut-offs, is essential in diagnosing insulin dysregulation, a common equine endocrinopathy. However, different insulin assays provide disparate results. We aimed to ease comparison between assays by compiling original and published data into a web app to convert insulin measurements from one assay to another. Data were available for ADVIA Centaur insulin chemiluminescent immunoassay (CLIA), Beckman Coulter insulin radioimmunoassay (RIA), Immulite 1000 CLIA, Immulite 2000 CLIA, Immulite 2000 XPi CLIA, Mercodia equine insulin enzyme-linked immunosorbent assay (ELISA), and Millipore porcine insulin RIA. Linear models were fitted for 13 assay pairs using non-decreasing splines, and integrated into this app. Assay comparisons including data from several studies showed a lower performance. This indicates technical variation between laboratories, which has not been described before, but is relevant when diagnostic measurements and cut-offs are provided by different laboratories. Nevertheless, the models' overall high performance (median r2 = 0.94; range 0.57-1.00) supports their use to interpret results from diagnostic insulin measurements when the reference assay is unavailable, and to compare values obtained from different assays.

Equine Endocrine Disease: Challenges With Case Definition for Research

Equine endocrine disease is an important area for equine research, requiring an appropriate case definition for inclusion and criteria for exclusion from disease. Defining a case for research may be different from criteria for clinical diagnosis. Further, clinical diagnosis recommendations have been changing regularly, making this area challenging for equine scientists. This review discusses the diagnosis of major equine endocrine diseases, pituitary pars intermedia dysfunction, equine metabolic syndrome and insulin dysregulation, focusing on the most appropriate diagnostic methods for research case definitions. Different diagnostic methods, including use of reference intervals and clinical decision limits, will be discussed with their relative merits for use in case definition for research.

Investigating the Relationship Between Cardiac Function and Insulin Sensitivity in Horses: A Pilot Study

Metabolic syndrome in humans is commonly associated with cardiovascular dysfunction, including atrial fibrillation and left ventricular diastolic dysfunction. Although many differences exist between human and equine metabolic syndrome, both of these conditions share some degree of insulin resistance. The aims of this pilot study were to investigate the relationship between insulin sensitivity and cardiac function. Seven horses (five mares, two geldings, aged 17.2 ± 4.2 years, weight 524 ± 73 kg) underwent insulin-modified frequently sampled intravenous glucose tolerance testing to determine insulin sensitivity (mean 2.21 ± 0.03 × 10−4 L/min/mU). Standard echocardiograms were performed on each horse, including two-dimensional, M-mode, and pulse-wave tissue Doppler imaging. Pearson and Spearman correlation analyses were used to determine the association of insulin sensitivity with echocardiographic measures of cardiac function in 5 horses. Insulin sensitivity was found to be significantly correlated with peak myocardial velocity during late diastole (r = 0.89, P = 0.0419), ratio between peak myocardial velocity in early and late diastole (r = −0.92, P = 0.0263), isovolumetric relaxation time (r = −0.97, P = 0.0072), and isovolumetric contraction time (ρ = −0.90, P = 0.0374). These preliminary data suggest that decreased insulin sensitivity is correlated with alterations in both systolic and diastolic function, as measured with tissue Doppler imaging (TDI). Due to the small sample size of this study, the relationship between insulin sensitivity and myocardial function in horses requires further investigation.