Researchers ran four tests on the same group of horses to compare results and determine sensitivity and specificity.
[ IMG SOURCE ]
Pick an acronym, any acronym. Veterinarians seeking to confirm whether a horse has insulin resistance (IR) have several tests from which to select—generally identified by a lineup of letters—though one particular approach is currently the most common for IR testing. While researchers have evaluated several of the tests for quantifying systemic IR, until recently no one had compared the tests for agreement, or consistency of results.
“We don’t know how these tests compare in the same group of horses,” said Laura Dunbar, DVM, an internal medicine resident at The Ohio State University’s College of Veterinary Medicine, in Columbus, referring to the variety of horse populations in previous research. She and her colleagues evaluated four tests in 12 healthy adult light-breed horses, and she presented their results at the 2015 American College of Veterinary Internal Medicine Forum, held in Indianapolis in June.
At its simplest, IR is a reduction in insulin sensitivity that makes it more difficult for cells to take up blood sugar for metabolism or storage. Insulin resistance is important because aside from being harmful to the horse’s body—the condition is often likened to “pre-diabetes” in humans, a condition that precedes the onset of Type II diabetes—researchers have linked it to the development of laminitis. Dunbar and team compared four common methods of insulin sensitivity testing: basal serum insulin and glucose concentration, the oral sugar test (OST), the combined glucose-insulin test (CGIT), and the insulin-modified frequently sampled intravenous glucose test (FSIGTT). The veterinary community often employs a combination of basal insulin/glucose and the OST—the most practical evaluations.
The researchers found a lot of variation in how the tests classified the horses, Dunbar said. The FSIGTT classified seven as IR and five as insulin sensitive (IS, normal). Basal insulin and OST classified all horses as IS. One cut-off from the CGIT classified three horses as IS and nine horses as IR, while the other cut-off classified 10 horses as IS and two as IR.
Dunbar said sensitivity of the CGIT using the insulin concentration at 45 minutes was 28.5% and specificity was 100%. (A highly sensitive test produces very few false negative results, and a highly specific one produces very few false positives.)
Finally, one statistical method for looking at relationships between the four tests showed that the tests were well-correlated, but another showed lack of agreement in their diagnostic accuracy (meaning that while the results of the tests might change in parallel—if the results of one test go up, so do the results of the other—this might not translate to the tests coming to the same conclusion of IR or IS, which also depends on where the cutoff values are set).
“Current tests (basal insulin concentrations and the OST) are highly specific but lack sensitivity,” she said. “They are useful screening tests (for severely IR individuals) but may miss mildly affected horses.”
Dunbar acknowledged more liberal criteria might be necessary to improve agreement; in other words, cutoffs might need to be changed slightly to reflect cases.
Questions and comments from the audience confirmed that, indeed, different opinions exist over what cutoffs confirm agreement between tests. One researcher pointed out that the scientific community will never agree on a cutoff.
“This study points out that the tests currently being used by veterinarians may classify horses very differently in terms of their insulin resistance,” Dunbar said. The researchers also noted that “further research is needed and will clarify the most clinically relevant cutoff values for these tests in horses. The veterinary community also looks forward to the results of research establishing values for these tests that are predictive of laminitis risk to more effectively guide patient management.”
Additionally:
Tests for Insulin Resistance
Basal serum insulin and glucose concentration Just like it sounds, this is simply a blood test that measures both glucose and insulin levels in the blood after a period of fasting. Everything from stress and time of day (hormone levels naturally fluctuate) to time of year/season and the horse’s diet/most recent meal can impact these levels, so veterinarians don’t generally use them to definitively diagnose the horse.
Oral sugar test This method involves collecting a blood sample during the morning hours, followed by administering light Karo syrup orally and collecting another blood sample 60 to 90 minutes later. Blood samples are evaluated for insulin levels.
Combined glucose-insulin tolerance test (CGIT) This approach involves the veterinarian administering glucose and insulin intravenously, then measuring blood glucose and insulin levels over the next several hours.
Insulin-modified frequently sampled intravenous glucose test (FSIGTT) This approach is similar to the CGIT, but involves administering glucose followed by insulin 20 minutes later. Blood glucose and insulin levels are measured very frequently after administering the glucose and insulin (ranging from every minute to every 5 to 10 minutes) over several hours. Modeling software is then used to calculate accurate estimates of insulin sensitivity.
- STEPHANIE L. CHURCH
