Evaluation of a commercially available radioimmunoassay and species-specific ELISAs for measurement of high concentrations of insulin in equine serum

Objective-To evaluate a human radioimmunoassay (RIA) and equine and high-range porcine (hrp) species-specific ELISAs for the measurement of high serum insulin concentrations in ponies. Samples-Serum samples from 12 healthy nonobese ponies (7 clinically normal and 5 laminitis prone; 13 to 26 years of age; 11 mares and 1 gelding) before and after glucose, insulin, and dexamethasone administration. Procedures-Intra- and interassay repeatability, freeze-thaw stability, dilutional parallelism, and assay agreement were assessed. Results Assay detection limits were as follows: RIA, < 389 mu U/mL; equine ELISA, < 175 mu U/mL; and hrp ELISA, 293 to 8,775 mu U/mL. Mean +/- SD intra- and interassay repeatability were respectively as follows: RIA, 6.5 +/- 5.1% and 74 +/- 3.4%; equine ELISA, 10.6 +/- 11.0% and 9.0 +/- 4.6%; and hrp ELISA, 19.9 +/- 17.2% and 17.3 +/- 16.6%. Freezing and thawing affected measured concentrations. Dilutional parallelism in the RIA was only evident when insulin-depleted equine serum was used as a diluent (percentage recovery, 95.7 +/- 27.4%); in the ELISAs, dilutional parallelism was observed when a zero calibrator was used. Agreement between RIA and equine ELISA results was good for samples containing concentrations < 175 mu U of insulin/mL (bias, -18.5 +/- 25.5 mu U/mL; higher in RIA). At higher concentrations, assay agreement was poor between RIA and equine ELISA results (bias, -185.3 +/- 98.7 mu U/mL) and between RIA and hrp ELISA results (bias, 25.3 +/- 183.0 mu U/mL). Conclusions and Clinical Relevance-Agreement among results of the 3 assays was variable, and dilutional parallelism was only evident with the RIA when insulin-depleted equine serum was tested. Caution is recommended when evaluating high insulin concentrations measured with the RIA or ELISAs. (Am J Vet Res 2012;73:1596-1602)