Use of the oral sugar test in ponies when performed with or without prior fasting

Abstract

Summary Background It is recommended that the oral sugar test (OST) for insulin dysregulation (ID) be performed after an overnight fast, but fasting is impractical in ponies kept solely at pasture. There are few data on OST repeatability and reliability in ponies. Objectives To report 1) whether OST results obtained in the morning after an overnight fast or without fasting in the afternoon (FASTING/FED) can be used interchangeably, 2) time of highest insulin concentration Tmax[insulin], repeatability and reliability of insulin response to the OST when FASTING or FED and 3) dichotomous agreement (ID/normal) within a small sample when FASTING or FED. Study design Method comparison study. Methods Oral sugar tests were performed on four occasions in 10 adult native British ponies, twice FASTING and twice FED. Insulin concentrations were measured by radioimmunoassay at 0–120 min (T0,30,60,75,90,120). Differences between FASTING and FED results were assessed using mixed effects models. Indices of repeatability and reliability were calculated; dichotomous agreement was reported using kappa statistics. Results Serum [insulin] was significantly (P≤0.05) higher at T60–T90 with prior fasting (estimated differences [95% confidence intervals]): T60: 23.5 μiu/ml (8.7–38.4 μiu/ml), T75: 27.1 μiu/ml (12.3–41.8 μiu/ml), T90: 15.1 (0.36–29.9 μiu/ml). Most frequently, Tmax[ins] occurred at T30. At any single time point, within‐subject coefficients of variation were: FASTING: 40% and FED: 31%. The 95% limits for repeatability were FASTING: 29–340%, FED: 41–240%. Test reliabilities were FASTING: 0.70 and FED: 0.67. For dichotomous interpretation similar results (kappa = 0.7) were obtained using cut‐offs of [Insulin] >60 μiu/ml at T60 or T90 for FASTING and [Insulin] >51 μiu/ml at T30 or T60 for FED samples. Main limitations Oral sugar tests were performed on a small number of animals on one pasture during one season (spring). Conclusions Clinicians should beware of interpreting changes in absolute OST results owing to poor repeatability. When stabling is unavailable, OSTs of ponies at pasture may yield similar dichotomous results without prior fasting.

Repeatability of Oral Sugar Test Results, Glucagon‐Like Peptide‐1 Measurements, and Serum High‐Molecular‐Weight Adiponectin Concentrations in Horses

“…Results of our study indicate that there is good agreement in binary results for insulin concentrations (κ = 0.65 and 0.73 at 60 and 75 minutes, respectively) when the OST is repeated in the same horse. Our results compare favorably with another study that found that the OST had good repeatability (κ = 0.7) when binary (positive/negative) results were examined using an insulin cutoff concentration of 60 μU/mL at 60 and 90 minutes in fasted ponies. Bland‐Altman analysis did not identify any systemic difference in the results obtained from the 2 OSTs, because mean bias values were close to zero for all of the variables evaluated.…”

“…Testing was performed the following morning, but start times varied among horses and this may have affected results. Insulin concentrations were higher when the OST was performed under fasting conditions in healthy ponies, whereas others were not able to detect an effect of fasting on OST results in horses …”

“…Further refinements to the OST may be required to improve test performance, such as increasing the amount of corn syrup administered or measuring glucose and insulin concentrations at an earlier time point. Another study found that maximum insulin concentrations were detected more frequently at 30 minutes, rather than 60 or 90 minutes in healthy ponies, and this time point warrants further investigation in horses. In our study, 60‐ and 75‐minute time points were selected, instead of 60 and 90 minutes as described when the OST was introduced .…”

“…When repeatability of oral glucose tests has been examined, the in‐feed OGT was found to have reasonable repeatability, with median (range) coefficient of variation (CV) values of 23% (16–44%), 19% (31–43%), and 45% (23–75%) detected at 90, 120, and 180 minutes, respectively, for 8 healthy ponies undergoing 3 consecutive tests . Repeatability of the OST also has been assessed in healthy ponies, with results compared between tests performed twice, under fed or fasted conditions . The OST had good repeatability (κ = 0.7) when binary (positive/negative) results were examined using an insulin cutoff concentration of 60 μU/mL at 60 and 90 minutes, and higher insulin concentrations were detected when ponies were fasted before testing.…”

Phenotypic, hormonal, and clinical characteristics of equine endocrinopathic laminitis

“…), being a breed at‐risk of developing EMS, a history of being an easy‐keeper, founder (divergent growth) rings in 1 or more hoof wall, a familial history of laminitis, a dysregulated postprandial serum insulin response to an oral glucose test (OGT) or an oral sugar test (OST), with the choice of dynamic test dependent on the clinician's preference. When this test was performed, a diagnostic cutoff of >60 μIU/mL 60 minutes after the meal was used for the OST, whereas a cutoff value of >80 μIU/mL 2 hours after the meal was used for the OGT …”