Historically, Phoenix Lab submitted samples for equine insulin testing to an outside reference laboratory that uses the Immulite 1000 analyzer. We recently acquired the Immulite 2000 XPI, the most updated and current immunoassay analyzer on the market. After extensive testing and validation of equine insulin on the 2000 XPI and subsequent consultations with our pathology group and Dr. Rothschild, a leading equine internal medicine consultant, we are now able to offer equine insulin testing in-house at Phoenix Lab. As a result of our validation process, we found that the results from the more advanced 2000 XPI analyzer correlated closely to the Immulite 1000, but with a slight positive bias (results ran slightly higher). To account for this difference, we have established new normal reference interval utilizing data from 132 healthy horses, which are similar to other published reference intervals but lower than the Immulite 1000. Please keep this in mind when comparing insulin results prior to September 2018. Also, note that hemolyzed samples may give falsely low results and icteric and lipemic samples may give erroneous results. Hemolysis, lipemia and icterus should be avoided.
Insulin and insulin dysregulation: Insulin is the hormone that allows for glucose to enter various cells in the body. After a meal or when glucose concentrations are high (sometimes secondary to stress/high cortisol, medications etc), insulin is released by the pancreas. Abnormalities of insulin metabolism include hyperinsulinemia and insulin resistance and are collectively referred to as insulin dysregulation. Insulin dysregulation is a key component of equine metabolic syndrome, a collection of endocrine and metabolic abnormalities associated with the development of laminitis in equine species and may or may not be associated with Equine Cushing’s disease. Testing for both of these conditions is recommended depending on the age and clinical signs present. In cases where both are present, these must be treated individually so that insulin dysregulation can be successfully corrected.
Diagnosis of Insulin dysregulation: As with all laboratory testing, results need to be interpreted in light of clinical signs, current health status and any medications that the horse may be on. If insulin dysregulation is suspected, you can start with baseline/resting insulin concentrations. It is ideal that the horse be sampled in the morning and the blood drawn prior to any stressors (including if there is active laminitis). They should be off pasture and fed no grain for 4 -12 hours (see Table I and Table II) and no tranquilizers given. Note that all insulin samples are run with glucose at no extra cost. For the insulin test, a tiger top tube or red top tube is required. For an accurate glucose[CR1] result, a grey top tube with at least 1 ml of blood is ideal (these can be obtained from Phoenix at no cost). If not using a grey top tube, the serum needs to be separated from the red cells within 20-30 minutes of collection. Your serum sample must be free from hemolysis (place the tube in a rack, in a chilled cooler, but not directly on ice, or in a rack in a refrigerator), lipemia and icterus. If there is going to be a submission delay of >24 hours, the serum should be frozen to preserve the insulin.
Resting insulin concentrations – usually only identifies the severely affected horses
- Table I – From the 2016 recommendations of the Equine Endocrine group https://sites.tufts.edu/equineendogroup/files/2016/11/2016-11-2-EMS-EEG-Final.pdf (no grain or lush grasses within 4 hours and hay is OK):
- Table II – From Dr. Frank’s paper (2008) http://www.lloydinc.com/media/filer_private/2018/01/17/guide_insulin_resistance_08_2.pdf, (no grain or lush green grass for 12 hours and hay is OK)
*** Horses with Equine Cushing’s Disease/PPID may or may not have accompanying insulin dysregulation. Thus, the fact that there is no insulin dysregulation does not mean the horse does not have Equine Cushing’s disease/PPID. In all cases where the patient is over 10 years of age and has any clinical sign of polyuria/polydipsia, unusual shedding and/or sweating patterns, is lethargic or poor doing, has recurrent hoof abscesses as well as several other potential indicators of the disease it is recommended they also be tested for it via the PRE and POST ACTH-TRH stimulation test.
For a more detailed discussion with hyperlinks to these references and for information about dynamic testing, please visit our website at http://phoenixlab.com/wp-content/uploads/2018/09/equineinsulin.pdf.
If you have any questions, or would like to speak to our equine internist, Dr Rothschild, please call 1-800-347-0043.