A sodium:potassium ratio combined with white blood cell counts may hold the key to diagnosis.
Dogs with hypoadrenocorticism can have a range of clinical presentations that mimic other disease processes, so effective screening is beneficial when you suspect this endocrinologic disorder in a patient. Hypoadrenocorticism is diagnosed based on the results of an ACTH stimulation test. However, this test is not routinely performed unless there is a high level of suspicion.
A basal serum cortisol concentration is a simpler, less expensive screening tool. This method is highly sensitive but has low specificity and, thus, limited usefulness. A serum sodium:potassium ratio < 27 is suggestive of a mineralocorticoid deficiency but is not a highly sensitive or specific test for hypoadrenocorticism. Likewise, leukocyte parameters for dogs with hypoadrenocorticism generally fall within the normal reference ranges.
What they did
To help find an effective screening tool, researchers at the University of Pennsylvania recently completed a retrospective, case-controlled study in an effort to clarify the value of a sodium:potassium ratio combined with white blood cell counts in screening for hypoadrenocorticism.
Study inclusion criteria
Included in the study were 53 dogs definitively diagnosed with hypoadrenocorticism and 110 control dogs.
Some noteworthy inclusion criteria and study interpretation limitation notes can be found in the article itself.
What they found
The sodium:potassium ratio; hematocrit; neutrophil, lymphocyte, and eosinophil counts; and neutrophil:lymphocyte ratio were all significantly different between dogs with and without hypoadrenocorticism, but the sensitivity and specificity were superior for the sodium:potassium ratio and lymphocyte count.
Furthermore, the results suggest that a patient’s absolute lymphocyte count at the time of initial evaluation represents an additional screening test that is more specific than the sodium:potassium ratio. Dogs with hypoadrenocorticism had significantly higher lymphocyte counts, although most were still within the reference range.
The finding that the sodium:potassium ratio is a less sensitive test than the lymphocyte count is consistent with previous research showing that electrolyte abnormalities, representing mineralocorticoid deficiency, occur in only 76% of hypoadrenocorticism cases.
Practitioners will want to consider that this study indicates that using both the lymphocyte count and the sodium:potassium ratio as a screening test for dogs with hypoadrenocorticism is a superior method than using either test by itself.
Seth M, Drobatz KJ, Church DB, et al. White blood cell count and the sodium to potassium ratio to screen for hypoadrenocorticism in dogs. J Vet Intern Med 2011;25(6):1351-1356.