Although various methods have been developed for diagnosing hyperadrenocorticism in dogs, ACTH stimulation testing is preferred in certain situations.1 In addition, ACTH stimulation testing is the only method of documenting hypoadrenocorticism in dogs and for assessing the response to mitotane, trilostane, or ketoconazole treatment in dogs with hyperadrenocorticism.
To date, the only commercially available form of ACTH proven to be effective for stimulation testing of dogs with adrenal gland disease is cosyntropin. Because the cost of this drug is so high, using a reduced dose would be helpful in decreasing the overall expense of ACTH stimulation testing. However, for reliable diagnosis of hyperadrenocorticism, a dose high enough to provide maximal adrenal gland stimulation must be used. Previous studies2–4 have found that IV administration of cosyntropin at a dose of 5 mg/kg (2.3 mg/lb) provides maximal adrenal gland stimulation, but when the IM route of administration is used, only a dose of 250 mg has been verified to provide maximal stimulation.5 Intramuscular administration could be advantageous in dogs in which IV injection is difficult, such as small or aggressive dogs, and in dogs that weigh < 50 kg (110 lb), use of a dose of 5 mg/kg would be < 250 mg, providing an economic benefit.
The purpose of the study reported here, therefore, was to compare adrenal gland stimulation achieved following administration of cosyntropin at a dose of 5 mg/kg, IM, with stimulation achieved following administration of the same dose IV.
Materials and Methods
Nine healthy mixed-breed dogs and 9 dogs with hyperadrenocorticism were used in the study. The healthy dogs consisted of 3 neutered males, 3 spayed females, and 3 sexually intact females. These dogs weighed between 7.5 and 29.3 kg (16.5 and 64.5 lb) and were considered healthy on the basis of results of a physical examination and routine laboratory testing (ie, a CBC, serum biochemical profile, and heartworm test). The 9 dogs with hyperadrenocorticism ranged from 9 to 15 years old and consisted of 3 Dachshunds, 2 Miniature Schnauzers, a Standard Poodle, a Miniature Poodle, and a mixed-breed dog. Hyperadrenocorticism had been diagnosed on the basis of history and results of a physical examination and routine laboratory testing; the diagnosis had been confirmed by means of ACTH stimulation testing with cosyntropin (5 mg/kg, IV) or low-dose dexamethasone suppression testing and on the basis of resolution of clinical signs with treatment for hyperadrenocorticism. The study protocol was reviewed and approved by the Auburn University Institutional Animal Care and Use Committee.
In the healthy dogs, ACTH stimulation testing was performed twice, with a minimum of 6 days between tests. For both tests, dogs were given cosyntropina at a dose of 5 mg/kg. However, for 1 test, cosyntropin was given IM, and for the other, it was given IV. Dogs were randomly assigned to receive cosyntropin IM or IV first. Intravenous injections were given into a cephalic or saphenous vein, and IM injections were given into the epaxial musculature. Blood was collected before (time 0) and 30, 60, 90, and 120 minutes after injection of cosyntropin.
In the dogs with hyperadrenocorticism, ACTH stimulation was also performed twice, but in all dogs, cosyntropin was administered IV during the first test. The second test was performed 7 to 11 days later, and cosyntropin was administered IM. Blood was collected before and 60 minutes after injection of cosyntropin.
Blood samples were allowed to clot and then centrifuged. Serum was harvested and stored at −20° C until analysis. Samples were assayed in duplicate with a previously validated radioimmunoassay kit.6,b All samples from the healthy dogs were assayed in a single batch; samples from the dogs with hyperadrenocorticism were assayed in a second batch. Sensitivity of the assay was 14 nmol/L.
Statistical analysis—For each group (ie, healthy dogs and dogs with hyperadrenocorticism), data were analyzed by means of repeated-measures ANOVA on ranks, followed by the Student-Newman-Keuls method. Statistical analyses were performed with standard software.c Values of P < 0.05 were considered significant.
Results
In the healthy dogs, serum cortisol concentration increased significantly (P < 0.001) over time after administration of cosyntropin, regardless of whether cosyntropin was administered IM or IV (Figure 1), and serum cortisol concentration was significantly increased, compared with baseline (time 0) concentration, at all times after cosyntropin administration. Serum cortisol concentration was highest 60 or 90 minutes after cosyntropin administration, regardless of route of administration, and for both routes of administration, serum cortisol concentrations 60 and 90 minutes after cosyntropin administration were not significantly different. When cosyntropin was given IV, serum cortisol concentration was higher at 60 than at 90 minutes after administration in 6 of 9 dogs. When cosyntropin was given IM, serum cortisol concentration was higher at 60 than at 90 minutes after administration in 5 of 9 dogs. Although there was no significant difference between serum cortisol concentrations at 60 and 90 minutes, individual dogs had great variation in concentrations measured at these 2 time points. In general, however, if maximum concentration occurred at 90 minutes for an individual dog, there was little difference between concentrations measured at 60 and 90 minutes, with the largest difference being 43 nmol/L. In contrast, in some dogs in which maximum concentration occurred at 60 minutes, the concentration measured at 90 minutes was > 100 nmol/L less. For both routes of administration, serum cortisol concentrations 30 and 120 minutes after cosyntropin administration were significantly lower than concentrations measured at 60 and 90 minutes. Serum cortisol concentrations measured after IM administration of cosyntropin were not significantly different from concentrations measured after IV administration at any time point.
In dogs with hyperadrenocorticism, serum cortisol concentration was significantly (P < 0.001) increased, compared with baseline concentration, 60 minutes after administration of cosyntropin, regardless of whether cosyntropin was administered IM or IV (Figure 2), and concentrations measured after IM administration of cosyntropin were not significantly different from concentrations measured after IV administration. For dogs in which serum cortisol concentration remained within reference limits after cosyntropin administration, similar responses were seen regardless of route of cosyntropin administration. In these dogs, hyperadrenocorticism was diagnosed on the basis of clinical signs, results of low-dose dexamethasone suppression testing, and response to treatment for hyperadrenocorticism.
Mean serum cortisol concentrations in 9 healthy dogs before (time 0) and after administration of cosyntropin at a dose of 5 mg/kg (2.3 mg/lb), IM and IV. Error bars represent SD. Values indicated by different letters were significantly (P < 0.05) different.
Citation: Journal of the American Veterinary Medical Association 229, 4; 10.2460/javma.229.4.528
Mean serum cortisol concentrations in 9 healthy dogs before (time 0) and after administration of cosyntropin at a dose of 5 mg/kg (2.3 mg/lb), IM and IV. Error bars represent SD. Values indicated by different letters were significantly (P < 0.05) different.
Citation: Journal of the American Veterinary Medical Association 229, 4; 10.2460/javma.229.4.528
Mean serum cortisol concentrations in 9 healthy dogs before (time 0) and after administration of cosyntropin at a dose of 5 mg/kg (2.3 mg/lb), IM and IV. Error bars represent SD. Values indicated by different letters were significantly (P < 0.05) different.
Citation: Journal of the American Veterinary Medical Association 229, 4; 10.2460/javma.229.4.528
Box plots of serum cortisol concentrations in 9 dogs with hyperadrenocorticism before and 60 minutes after administration of cosyntropin at a dose of 5 mg/kg, IV (A) and IM (B). For each plot, the box represents the interquartile (ie, 25th to 75th percentile) range, the line within the box represents the median, the horizontal bars represent the range for most of the data, and the circles represent outlying data points.
Citation: Journal of the American Veterinary Medical Association 229, 4; 10.2460/javma.229.4.528
Box plots of serum cortisol concentrations in 9 dogs with hyperadrenocorticism before and 60 minutes after administration of cosyntropin at a dose of 5 mg/kg, IV (A) and IM (B). For each plot, the box represents the interquartile (ie, 25th to 75th percentile) range, the line within the box represents the median, the horizontal bars represent the range for most of the data, and the circles represent outlying data points.
Citation: Journal of the American Veterinary Medical Association 229, 4; 10.2460/javma.229.4.528
Box plots of serum cortisol concentrations in 9 dogs with hyperadrenocorticism before and 60 minutes after administration of cosyntropin at a dose of 5 mg/kg, IV (A) and IM (B). For each plot, the box represents the interquartile (ie, 25th to 75th percentile) range, the line within the box represents the median, the horizontal bars represent the range for most of the data, and the circles represent outlying data points.
Citation: Journal of the American Veterinary Medical Association 229, 4; 10.2460/javma.229.4.528
Discussion
The goal in ACTH stimulation testing is to provide sufficient ACTH to induce maximal cortisol secretion by the adrenal glands. The time that the highest circulating concentration of cortisol occurs relative to ACTH administration may vary, depending on the form of the hormone used and its route of delivery.3–8 The time-dependent response of circulating cortisol concentration after administration of cosyntropin has been studied previously, as has the effect of varying the dose of ACTH.3–8 Results of the present study indicate that administration of cosyntropin at a dose of 5 mg/kg, IM, induces maximal adrenal gland response 60 minutes after administration.
Previous studies revealed that administration of 250 mg of cosyntropin IV or IM to healthy dogs induces maximal adrenal gland stimulation5 and that administration of cosyntropin at a dose of 5 mg/kg, IV, to healthy dogs or dogs with adrenal gland disorders induces maximal adrenal gland stimulation.2–4 However, the effects of administering cosyntropin at a dose of 5 mg/kg, IM, have not, to our knowledge, been examined previously. In the present study, administration of cosyntropin at a dose of 5 mg/kg, IM, resulted in serum cortisol concentrations 30, 60, 90, and 120 minutes after administration that were similar to concentrations obtained after IV administration at the same dose. In addition, serum cortisol concentrations in the present study were similar to those obtained in a previous study3 in which cosyntropin was administered at a dose of 5 mg/kg, IV. Serum cortisol concentration peaked 60 or 90 minutes after cosyntropin administration in the present study, and concentrations 60 and 90 minutes after cosyntropin administration were significantly higher than concentrations measured at other time points.
The ACTH stimulation test is often used to screen dogs for hyperadrenocorticism. On the basis of results for healthy dogs in the present study, we elected to measure serum cortisol concentration 60 minutes after cosyntropin administration in dogs with hyperadrenocorticism. Serum cortisol concentrations at this time were not significantly different between the 2 routes of administration. Thus, administration of cosyntropin at a dose of 5 mg/kg, IM, appears to be as effective as administration of the same dose, IV, in the diagnosis of hyperadrenocorticism in dogs. Use of this lower dose of cosyntropin for IM administration, compared with the previously recommended dose of 250 mg, IM, will be less expensive in most dogs. In addition, use of the IM, versus the IV, route of administration will be easier in small or aggressive dogs and in dogs with poor venous access.
Cortrosyn, Amphastar, Rancho Cucamonga, Calif.
Coat-a-Count cortisol assay, Diagnostic Products Corp, Los Angeles, Calif.
SigmaStat for Windows, version 1.0, Jandel Scientific, SPSS Inc, Chicago, Ill.
References
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