A sodium-to-potassium (Na:K) ratio of 27 has typically been used as a screening tool for hyperadrenocorticism (HA). A study was conducted to investigate the sensitivity and specificity of the Na:K ratio as a diagnostic aid and to determine whether there were any associations between venous pH and the Na:K ratio, plasma ionized calcium (iCa2+) concentration, or ionized magnesium (iMg2+) concentration in dogs with HA. Dogs were included if an adrenocorticotropic hormone (ACTH)-stimulation test confirmed a diagnosis of HA and the dog had a serum sodium concentration below the reference range or a serum potassium concentration above the reference range. Results indicated that the sensitivity and specificity of an Na:K ratio cutoff of 28 were 93% and 96%, respectively, with 95% of dogs correctly classified as diseased or not diseased. The sensitivity and specificity of an Na:K ratio cutoff of 27 were 89% and 97%, respectively, with 95% of dogs correctly classified. The sensitivity of an Na:K ratio of 30 was 100%, and the specificity was 84%; the sensitivity of an Na:K ratio of 24 was 79%, with a specificity of 100%. These results suggest that an ACTH-stimulation test in dogs with an Na:K ratio < 24 is likely to confirm the diagnosis of HA. The authors also concluded that ACTH-stimulation tests in dogs with an Na:K ratio between 27 and 30 could be beneficial, even though most dogs with HA have a lower Na:K ratio. The likelihood of diagnosing HA in a dog with an Na:K ratio > 30 is low. An association between decreased venous pH and hyperkalemia was identified, suggesting that hyperkalemia impacts acid-base status in dogs with HA. In addition, these data confirmed that ionized hypercalcemia develops in dogs with naturally occurring HA and that serum iCa2+ concentration is inversely associated with venous pH.

COMMENTARY: The sensitivity and specificity data for different Na:K ratios presented in this study are interesting and help to confirm the conventional recommendation that an Na:K ratio cutoff value of 27 or 28 can be used to identify dogs that would benefit from further testing for hypoadrenocorticism. Although an Na:K ratio < 24 had 100% specificity (therefore no false-positive results) in this study, practitioners must keep in mind that numerous other causes of such low Na:K ratios have been documented, including anuric/oliguric renal failure, ruptured bladder or urinary tract obstruction, intestinal parasites, and intestinal salmonellosis, just to name a few. It should also be remembered that some dogs with hypoadrenocorticism do not have the classic electrolyte abnormalities. Thus, suspected hypoadrenocorticism must always be confirmed with an ACTH-stimulation test.

Abnormalities of serum electrolyte concentrations in dogs with hypoadrenocorticism. Adler JA, Drobatz KJ, Hess RS. J VET INTERN MED 21:1168-1173, 2007.