It has been established that high blood insulin is responsible for about 90% of all laminitis cases and best current estimate is 10 to 15% of the equine population is at risk of EMS – Equine Metabolic Syndrome. A central component of EMS is elevated insulin.
Not all overweight horses have EMS
Some (not all) EMS horses are overweight and have abnormal fat deposits, especially along the crest of the neck. These are indicators but to confirm 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 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 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 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 nonfasting insulin with the horse having access to hay or pasture only. However, instead of the 20 uIU/mL upper limit of normal currently recommended we use the ranges established by the large pony field trial. So far, we see no reason to change.
Eleanor Kellon, VMD