German study finds shortcomings in insulin testing in horses

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Insulin test results in horses from commonly used laboratory tests should be interpreted carefully, according to German researchers, who found significant variations using three tests on the same blood samples.

Precise analysis of equine insulin in blood samples is the key element for assessing insulin resistance or insulin regulation problems in horses.

However, previous studies have indicated marked differences in insulin concentrations obtained from sample analyses with different immunoassays, which are lab tests that detect or measure specific proteins or other substances through their properties as antigens or antibodies.

Most immunoassays used in veterinary medicine for analyzing insulin concentrations in equine serum or plasma were originally designed for human diagnostics and research, and specific immunoassays for the quantifying equine insulin were not commercially available, Tobias Warnken and his colleagues noted in the journal BMC Veterinary Research.

Species-specific lab tests were being used more frequently and seemed to provide advantages compared to human-specific tests, they said.

The study team set out to compare three immunoassays: one porcine-specific insulin enzyme-linked immunosorbent assay (ELISA) advertised to be specific for equine insulin; one porcine-specific insulin radioimmunoassay (RIA); and one human-specific insulin chemiluminescence immunoassay (CLIA).

The researchers, from the University of Veterinary Medicine Hannover, Foundation, and the University of Hohenheim, noted that all three were widely used in veterinary laboratories for the analysis of equine insulin.

They tested their clinical applicability in assessing insulin resistance and insulin regulation problems by analysis of blood samples obtained during a dynamic diagnostic stimulation test involving elevated insulin concentrations.

Forty blood serum samples used in the study were collected from 17 horses and ponies of different breeds, some healthy and some diagnosed with insulin problems.

“Insulin values obtained from the ELISA, RIA and CLIA, investigated for analyses of basal blood samples differed significantly between all three assays [tests],” they reported.

“Analyses of samples obtained during dynamic diagnostic stimulation testing with consecutively higher insulin concentrations revealed significantly lower insulin concentrations supplied by the CLIA compared to the ELISA.

“However, values measured by ELISA were intermediate and not different to those measured by RIA.”

They continued: “Our results indicate that insulin concentrations of one sample measured by different methods vary greatly and should be interpreted carefully.”

The researchers said consideration of the immunoassay method and reliable test-specific reference ranges were of particular importance, especially in clinical cases where small changes in insulin levels can cause false classification in terms of insulin sensitivity of horses and ponies.

There was, they said, an urgent need to reevaluate the reference ranges established on analyses from samples measured with other approaches for measuring insulin.

Comparison of three different methods for the quantification of equine insulin
T. Warnken, K. Huber and K. Feige
BMC Veterinary Research 2016 12:196 DOI: 10.1186/s12917-016-0828-z

The study, published under a Creative Commons License, can be read here

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