The adrenal response to critical illness and its role in prognostication has been an area of intense study in human medicine. There is general consensus on the positive association of high basal cortisol and adverse outcome in human and canine medicine. The association of serum ACTH-stimulated cortisol and delta cortisol concentrations with outcome is however in a state of equipoise. Various delta cortisol cut-off values and definitions for adrenal insufficiency abound. A recent study documented relative adrenal insufficiency in a group of 33 septic dogs, without reporting the basal cortisol concentrations of the patient groups and despite finding no association between survival and delta cortisol as a continuous, non-dichotomized variable.

This prospective, case-controlled study sought to determine the association between adrenal function, severity of illness and outcome in dogs with naturally occurring babesiosis in an attempt to document adrenal insufficiency. Sixty-eight dogs with canine babesiosis were studied. Fifteen healthy dogs were used as controls. Blood samples were obtained from the jugular vein in each dog prior to treatment, for the measurement of basal serum cortisol concentrations. Immediately thereafter each dog was injected intravenously with 5 ug/kg of ACTH (tetracosactrin). A second blood sample was taken 1 hour later for serum ACTH-stimulated cortisol measurement and delta cortisol was calculated. Three outcomes were defined: hospitalised dogs that died, Group D (n = 4); hospitalised dogs that survived, Group S (n = 48); and dogs sent home, Group H (n = 16). Data was analysed by the Kruskal Wallis and Mann Whitney U tests for multiple and pair-wise comparisons, respectively. Significance was set at p < 0.05.

Median basal cortisol was significantly higher in patients than in controls (156 vs. 83 nmol/l), whereas ACTH-stimulated cortisol (346 vs. 322 nmol/l) was similar. Median delta cortisol was therefore lower in patients (185 vs. 239 nmol/l). ACTH-stimulated cortisol was significantly higher in Group D (510 nmol/l), compared to group S (347 nmol/l) and compared to group H (301 nmol/l). Delta cortisol concentrations tended to be lower in dogs with higher basal cortisol concentrations, demonstrating a significant negative correlation (r = -0.516, p <0.001).

Caution should be exercise in over interpreting low delta cortisol without due cognisance of the confounding effect of high basal cortisol, especially in very ill dogs, potentially resulting in erroneous and gratuitous use of corticosteroids.