How do I diagnose and manage insulin dysregulation in a horse with equine metabolic syndrome?

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Diagnosis of equine insulin dysregulation (ID) begins with a resting basal insulin measurement, followed by dynamic oral challenge testing when basal results are equivocal or when greater sensitivity is required. ID is defined as any combination of basal hyperinsulinemia, postprandial hyperinsulinemia on dynamic testing, or peripheral tissue insulin resistance, and is the central component of equine metabolic syndrome (EMS).113 ---  Diagnosis Basal insulin is the first-line screening test. Blood is collected from a horse that has been fasted for 8–10 hours, or at minimum has had hay withheld overnight.9 Serum or heparinized plasma is the preferred sample type; ethylenediaminetetraacetic acid (EDTA) tubes produce significantly lower insulin concentrations and should not be used.3 Samples analyzed within 3 days at 20°C or within 8 days at 4°C are stable; serum and heparinized samples perform equivalently within those windows.3 Basal insulin alone has lower sensitivity than dynamic testing for detecting ID, and a normal resting result does not exclude the diagnosis.13 Dynamic oral challenge testing is the preferred method for diagnosing ID and is more sensitive than basal testing alone. The oral sugar test (OST) and oral glucose test (OGT) are the most commonly used dynamic tests; both evaluate the insulin response to an oral carbohydrate stimulus and are considered equivalent for identifying horses with ID.38 The OST uses commercial corn syrup at doses of 0.15–0.45 mL/kg body weight administered to a fasted horse, with blood collected for insulin measurement at 60–90 minutes post-administration.1113 Higher doses have been proposed to improve diagnostic accuracy, though this remains controversial, and higher volumes may be impractical in larger horses.11 The OGT follows a similar principle using oral dextrose.8 Both tests require an 8–10 hour fast and approximately 2 hours to complete.9 The 2-step insulin sensitivity test evaluates peripheral tissue insulin resistance