Veterinary Medicine - June 2008 - (Page 326)

Canine hypoadrenocorticism PEER-REVIEWED nously or intramuscularly or whether samples are collected 60 or 90 minutes after cosyntropin administration.20 A post-ACTH plasma cortisol concentration < 2 µg/dl indicates a lack of an appropriate response and is diagnostic for hypoadrenocorticism.1 To minimize the cost of cosyntropin, an intravenous dose of 0.005 mg/kg may be used since this dose results in a similar degree of stimulation in healthy dogs and dogs with hyperadrenocorticism.19,21 Reconstituted cosyntropin can be stored frozen (-4 F [-20 C]) in plastic syringes for up to six months.22 Although no published data con rming this exist, some veterinarians store reconstituted cosyntropin in the refrigerator (39.2 F [4 C]) for up to 24 hours without a perceived change in potency. Basal cortisol concentration. Measuring a basal cortisol concentration has been proposed as a diagnostic test for hypoadrenocorticism in dogs. In one study of 13 dogs with hypoadrenocorticism and 110 healthy dogs, a basal cortisol concentration ≤ 1 µg/dl had a sensitivity of 100% and speci city of 98.2% for hypoadrenocorticism.23 The same study found that using a cutoff of ≤ 2 µg/dl for the basal cortisol concentration had a negative predictive value of 100%.23 These data indicate that basal cortisol concentrations can be used to rule out hypoadrenocorticism in dogs with appropriate clinical signs that are not receiving drugs that could potentially alter cortisol secretion such as glucocorticoids, mitotane, trilostane, or ketoconazole. However, given the low disease prevalence of hypoadrenocorticism in dogs, this test had a poor positive predictive value.23 Since basal cortisol concentrations have a poor positive predictive value and hypoadrenocorticism requires life-long therapy, ACTH stimulation testing is still the test of choice for de nitively diagnosing hypoadrenocorticism in dogs. CAR. The ratio of basal cortisol concentration to endogenous ACTH concentration (CAR) is an additional method to diagnose primary hypoadrenocorticism.24 The reference interval CAR in healthy dogs is 1.1 to 26.24 Dogs with primary hypoadrenocorticism should have low cortisol and high ACTH plasma concentrations, resulting in a low CAR. In one study of 22 dogs with primary hypoadrenocorticism, the CAR was 0.003 to 0.17, well below the reference interval.24 This test allows for a speci c diagnosis of primary hypoadrenocorticism in dogs with a single blood sample. Since endogenous ACTH concentrations will be low in dogs with secondary hypoadrenocorticism, CAR does not help differentiate normal dogs from dogs with secondary hypoadrenocorticism. Endogenous ACTH concentrations. After de nitive diagnosis, endogenous ACTH concentrations may be used to differentiate primary from secondary cortisol de ciency.1 Since dogs with primary glucocorticoid de ciency may develop a mineralocorticoid de ciency whereas dogs with secondary glucocorticoid de ciency should not, differentiating primary from secondary hypoadrenocorticism may help guide clinical monitoring.3,13 Dogs with pituitary dysfunction resulting in secondary hypoadrenocorticism should have low endogenous ACTH concentrations. On the other hand, dogs with primary hypoadrenocorticism should have a normally functioning pituitary gland, so their ACTH concentrations should be increased in response to hypocortisolemia.1 Mineralocorticoid deﬁciency Mineralocorticoid de ciency is easily diagnosed based on recognizing hyperkalemia and hyponatremia (Table 3).1 Dogs with a cortisol de ciency that lack hyponatremia and hyperkalemia do not have a clinically important mineralocorticoid de ciency, so additional diagnostic testing for aldosterone deciency is rarely indicated. Measuring the basal aldosterone con- 326 June 2008 VETERINARY MEDICINE

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Veterinary Medicine - June 2008 Contents Leading Off Clinical Exposures Idea Exchange A Challenging Case: Conjunctival Lymphoma in a Cat Updates on Canine Hypoadrenocorticism Product Preview CE Form Advertiser Index Marketplace/Classifieds Mind Over Miller Clinical Edge - June 2008 Table of Contents Managing Diabetes Mellitus in Cats: What Makes it Work? Porcine Insulin Zinc Suspension in the Management of Feline Diabetes Mellitus Monitoring Diabetes Mellitus in Diabetic Cats Feline Diabetes Mellitus: Important Client Communication Points

Veterinary Medicine - June 2008

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