The effect of starting time on dexamethasone suppression test results in horses

This study was conducted to investigate the effect of starting time on dexamethasone suppression test results in horses. Eight adult horses were used throughout the trial. Baseline cortisol levels were established by collecting cortisol levels twice daily, at 8:00 A.M. and 8:00 P.M. for 4 consecutive days. Morning baseline cortisol levels were 46.3 +/- 5.94 ng/ml, and evening baseline cortisol levels were 32.8 +/- 5.59 ng/ml. Although lower, the evening cortisol levels were not statistically different (P = 0.154) from the morning levels. Dexamethasone suppression tests initiated at either 9:00 A.M. or 9:00 P.M. were performed by collected a control blood sample, administering either 0.044 mg/kg dexamethasone or its vehicle intravenously and then collecting additional blood samples at 6, 12, 24, 36, and 48 hr after treatment. Mean cortisol levels at hr 0, 6, 12, 24, 36, and 48 after a dexamethasone injection given at 9:00 A.M. were 55.6 +/- 3.08, 6.4 +/- 2.05, 0.73 +/- 0.48, 11.0 +/- 5.82, 12.6 +/- 4.30, and 40.5 +/- 5.38 ng/ml respectively. Mean cortisol levels at hr 0, 6, 12, 24, 36, and 48 hr after a dexamethasone injection given at 9:00 P.M. were 45.0 +/- 6.03, 4.5 +/- 1.28, 0.20 +/- 0.12, 4.5 +/- 2.49, 23.4 +/- 5.88, and 29.5 +/- 6.61 ng/ml respectively. There was no statistical difference in cortisol values between A.M. and P.M. initiated tests at any hour post dexamethasone administration. There was no decrease in cortisol level after administration of dexamethasone vehicle.