Biological Variability in Serum Cortisol Concentration Post‐adrenocorticotropic Hormone Stimulation in Healthy Dogs

Abstract

BackgroundThe ACTH stimulation has low sensitivity for the diagnosis of hypercortisolism possibly as a result of biological and analytical variability.Hypothesis/ObjectivesTo report the components of biological and analytical variability in serum cortisol concentration post‐ACTH stimulation ([cortisol]) in healthy dogs.AnimalsFourteen healthy harrier hound dogs.MethodsThe data were extracted from a separate, prospective, randomized, double‐blinded, controlled discovery study in which dogs treated with vehicle control and 4 different doses of cortisone acetate (CA) for 7 days had an ACTH stimulation test performed to confirm the dose‐dependent effect of CA. The index of individuality (IoI), the critical difference between sequential measurements (C D), and the number of measurements required to assess the homeostatic set point (HSP) of [cortisol] with confidence intervals (CI) of 90 and 95% were estimated.ResultsThe IoI was equal to 1.1 and the C D was 3.3 μg/dL (92 nmol/L). The number of measurements required to assess the HSP of [cortisol] with CI of 90 and 95% were 3 and 15, respectively. Additionally, mean [cortisol] was higher in males than in females (13.3 ± 4 μg/dL [366 ± 114 nmol/L] vs. 11.5 ± 2.5 μg/dL [318 ± 65 nmol/L], respectively; P = .046). As expected, treatment with CA resulted in a dose‐dependent suppression of [cortisol].Conclusions and Clinical ImportanceFalse‐negative test results in hypercortisolism could occur when [cortisol] is outside of the individual's HSP and within the reference interval. The large C D emphasizes the importance of assessing clinically relevant parameters in the diagnosis and monitoring of HC.

Variability of Symmetric Dimethylarginine in Apparently Healthy Dogs

“…where Z X is the percentile of the standard normal distribution and D is the desired percentage closeness to the HSP (Z X 5 1.645 for X 5 90% and D 5 10%; Z X 5 1.96 for X 5 95% and D 5 5%). 19 We used the outliers package 23 cochran.test() function to test if any of the dogs was significantly influencing the variance from the other dogs and the BlandAltmanLeh package 24 bland.altman.plot() function to generate the Bland-Altman plot. We used the lmerTest package 22,25 lmer() function to perform a regression analysis for repeated measures, to evaluate the relationship between the fixed effect factor "day of sampling" and the dependent variables "SDMA" and "creatinine"; a random intercept was fitted for each dog.…”

“…The IOI was calculated as ( + ) 1/2 /CV G . The number of specimens that should be assayed to be X% confident of achieving an estimate of the HSP within D% of an individual dog was calculated from the formula n = Z X 2 ( + )/ D 2 , where Z X is the percentile of the standard normal distribution and D is the desired percentage closeness to the HSP ( Z X = 1.645 for X = 90% and D = 10%; Z X = 1.96 for X = 95% and D = 5%) . We used the outliers package cochran.test() function to test if any of the dogs was significantly influencing the variance from the other dogs and the BlandAltmanLeh package bland.altman.plot() function to generate the Bland–Altman plot.…”

“…Each 2 mL sample was obtained after gentle manual restraint (with the dog in a sitting position) and drawn via jugular venipuncture using a 20 G needle and a 3 mL syringe. b The blood samples were collected on Days 1,3,5,12,19,26,40,54, and 68. After collection, blood was transferred into serum tubes.…”

“…Several studies have investigated the TBV and clinical utility of various tests in veterinary medicine including; hematological variables, platelet function, hemostatic variables, cardiac troponin I, N‐terminal pro‐brain natriuretic peptide, serum cystatin C, C‐reactive protein, specific canine pancreatic lipase immunoreactivity, canine serum cortisol concentration post‐adrenocorticotropic hormone stimulation, and canine serum thyrotropin concentration …”

“…7,8 With such tests, it becomes important to calculate the critical difference (C D ) which is the minimum change necessary between sequential results for an individual to exceed the TBV of the test. 7 Several studies have investigated the TBV and clinical utility of various tests in veterinary medicine including; hematological variables, 11 platelet function, 12 14 N-terminal pro-brain natriuretic peptide, 14,15 serum cystatin C, 16,17 C-reactive protein, 18 specific canine pancreatic lipase immunoreactivity, 18 canine serum cortisol concentration post-adrenocorticotropic hormone stimulation, 19 and canine serum thyrotropin concentration. 20 Based on the current veterinary literature regarding SDMA, we hypothesized that SDMA would have a lower individuality than sCr because of less influence from extrarenal factors.…”