Urinary cortisol:creatinine ratios in healthy horses and horses with hyperadrenocorticism and non-adrenal disease

Urinary cortisol and creatinine concentrations, and the cortisol:creatinine ratio were compared between 12 healthy horses (group 1), 13 horses with Cushing's disease (group 2), and eight horses with dysautonomia syndrome (equine grass sickness) (group 3). The mean (sd) urinary cortisol concentrations were 112 (55.7), 250 (357) and 864 (526) nmol/litre in groups 1, 2 and 3, respectively; the mean (sd) urinary creatinine concentrations were 18.9 (7.3), 12.0 (6.7) and 45.2 (26.4) nmol/litre in groups 1, 2 and 3, respectively, and the mean (sd) ratios were 6.1 (2.6), 19.8 (23.8) and 21.3 (14.5) (x 10(-6)) in groups 1, 2 and 3, respectively. The urinary cortisol and creatinine concentrations were significantly greater in group 3 than in groups 1 and 2, but the ratios were not significantly different, although there was a trend (P=0.076) towards higher values in groups 2 and 3. A diagnostic cut-off in the cortisol:creatinine ratio for the confirmation of Cushing's disease of more than 6.9 x 10(-6) was associated with a diagnostic sensitivity and specificity of 92.3 and 75.0 per cent, respectively, when compared with healthy horses. However, when group 3 horses were included, a cut-off of more than 7.4 x 10(-6) was associated with a diagnostic sensitivity and specificity of 84.6 and 54.5 per cent, respectively.