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

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.

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 T_max [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.

Method comparison study.

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 (T_{0,30,60,75,90,120} ). 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.

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, T_max [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.

Oral sugar tests were performed on a small number of animals on one pasture during one season (spring).

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.