Comparison of a modified 2-step insulin response test performed with porcine zinc insulin and an oral glucose test to detect hyperinsulinemic Icelandic horses.
Authors: Frers F, Delarocque J, Huber K, Feige K, Warnken T
Journal: Veterinary journal (London, England : 1997)
Summary
# Editorial Summary Insulin dysregulation in horses manifests along a spectrum, with hyperinsulaemia and peripheral insulin resistance representing distinct pathological processes that may not always co-exist—a distinction that becomes critical when selecting appropriate diagnostic protocols. Frers and colleagues investigated whether the modified 2-step insulin response test (2-step IRT) using porcine zinc insulin could reliably detect hyperinsulinaemic horses by comparing it against the oral glucose test (OGT) in a crossover study of twelve Icelandic horses. Whilst the OGT successfully identified six horses as hyperinsulinaemic and six as normoinsulinaemic, the 2-step IRT classified all twelve horses as insulin resistant, with no significant differences in glucose reduction at 30 minutes between the two groups, and notably, none achieved the expected ≥50% glucose reduction threshold. The findings highlight a crucial diagnostic limitation: an intravenous insulin challenge may detect peripheral insulin resistance even in horses without demonstrable hyperinsulinaemia, meaning the choice of testing protocol fundamentally shapes clinical interpretation and potentially misguides management decisions. Practitioners should recognise that oral and intravenous tests answer different diagnostic questions, and selecting between them requires clarity about whether identifying hyperinsulinaemia (OGT) or peripheral insulin resistance (IV challenge) is most relevant to the individual horse's clinical presentation and metabolic risk profile.
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Practical Takeaways
- •OGT and IV insulin response tests detect different aspects of insulin dysregulation; OGT identifies hyperinsulinemia while IV tests identify peripheral insulin resistance — use both or choose based on clinical suspicion to avoid missing cases
- •A modified 2-step IRT with porcine zinc insulin may over-identify insulin resistance compared to oral testing; interpret results cautiously and consider combining tests for comprehensive metabolic assessment
- •Not all hyperinsulinemic horses demonstrate proportional peripheral insulin resistance, so a normal IV glucose clearance does not rule out metabolic dysfunction or laminitis risk in susceptible breeds like Icelandic horses
Key Findings
- •Oral glucose testing (OGT) identified 6 of 12 Icelandic horses as hyperinsulinemic, while the modified 2-step insulin response test (2-step IRT) with porcine zinc insulin identified all 12 horses as insulin resistant
- •Glucose reduction during the 2-step IRT was similar between hyperinsulinemic and non-hyperinsulinemic horses, with none achieving ≥50% glucose reduction at 30 minutes
- •Hyperinsulinemia and peripheral insulin resistance appear to be different aspects of insulin dysregulation that may not always overlap in individual horses
- •Choice of diagnostic testing protocol significantly impacts detection rates and clinical interpretation of insulin dysregulation in horses