Eleanor M. Kellon, VMD
Some, but not all, Equine Metabolic syndrome (EMS) horses are overweight and have abnormal fat deposits, especially along the crest of the neck. These are indicators, but to confirm a diagnosis of EMS, you need to document abnormal insulin.
If you have done any reading on EMS diagnostic tests, you have probably found many different testing options. A common statement these days is that the oral sugar test is best — but is it?
The original test suggested for documenting abnormally high insulin was a fasting insulin level. The oral sugar test (OST) was born after the realization that baseline insulin when fasting missed a lot of cases. There is no question it's better than a fasting insulin level, but is it the best and only option?
There are several multi-step tests for insulin function and response to insulin, which involve placement of a catheter, and evaluating the response to intravenous injection of glucose and/or insulin by taking a varying number of blood samples. Because of their expense and the time involved, simple testing involving minimal time, and preferably only one blood draw, are more appropriate for screening horses in the field.
The value of field screening tests is established by comparing the results to those obtained from the more intricate, but more accurate, intravenous tests. The "gold standard" test is the FSIGTT (frequently sampled intravenous glucose tolerance test). Dunbar, et al., 2016, showed the OST had a 0% sensitivity in detecting insulin resistance compared to FSIGTT when using the common cutoff for peak insulin of 60 uIU/mL. When the threshold was lowered to 45 uIU/mL, sensitivity improved to 14%. This means for every 100 positive horses detected by FSIGTT, only 14 were found with OST.
In 2005, Dr. Trieber's VA Poly Technical group used the FSIGTT to develop the proxies RISQI (reciprocal of the square root of insulin) and MIRG (modified insulin to glucose ration). These are calculations using insulin or insulin and glucose results from a single blood draw, to approximate the results from FSIGTT. When put to the test in a year-long study of 160 ponies being maintained on pasture, either proxy had a sensitivity of at least 64%. When the two were combined with one other marker of EMS, such as obesity, sensitivity rose to 74%. The proxies correctly predicted ponies at risk of laminitis in 84.6% of the cases.
The OST has also been tested to see if it could differentiate between ponies that had previously had laminitis versus those that never had laminitis. At the usually used dosage of 0.15 mL of Karo syrup/kg of body weight, there was no significant difference. There was also no significant difference between groups at 0.30 mL/kg of body weight. At 0.45 mL/kg there was a significant difference but this would amount to a bit over 4 oz for a 1000 lb horse. Only 8 ponies were used in this study and sensitivity compared to FSIGTT has not been determined. Cutoff values for determining a positive test have also not been established for this dosage.
Newer is not always better. The ECIR group https://ecir.groups.io/g/main has been using the proxies since 2006, and before that the G:I (glucose insulin ratio) proxy, which was also shown to have good correlation with FSIGTT in Trieber's original publication. Another option is to use non-fasting insulin, with the horse having access to hay or pasture only. However, instead of the 20 uIU/mL upper limit of normal currently recommended, the ECIR Group uses the ranges established by the large pony field trial.
So far, we see no reason to change.
For more information see the ECIR Group film Diagnosis. https://www.ecirhorse.org/video.php
About ECIR Group Inc.
Started in 1999, the ECIR Group is the largest field-trial database for PPID and EMS in the world and provides the latest research, diagnosis, and treatment information, in addition to dietary recommendations for horses with these conditions. Even universities do not and cannot compile and follow long term as many in-depth case histories of PPID/EMS horses as the ECIR Group.
In 2013 the Equine Cushing's and Insulin Resistance Group Inc., an Arizona nonprofit corporation, was approved as a 501(c)3 public charity. Tax deductible contributions and grants support ongoing research, education, and awareness of Equine Cushing's Disease/PPID and EMS.
THE MISSION of the ECIR Group Inc. is to improve the welfare of equines with metabolic disorders via a unique interface between basic research and real-life clinical experience. Prevention of laminitis is the ultimate goal. The ECIR Group serves the scientific community, practicing clinicians, and owners by focusing on investigations most likely to quickly, immediately, and significantly benefit the welfare of the horse.
Contact: Nancy Collins
Stock photo available on request