What is the recommended initial approach to differentiating diabetes insipidus from psychogenic polydipsia in a dog?

• A. Check urine specific gravity and serum osmolality; if still uncertain, perform a desmopressin response test to distinguish central from nephrogenic DI
• B. Administer a high-salt diet and measure urine volume
• C. Evaluate for polydipsia with owner diary alone
• D. Do a complete blood count and chemistry panel

Answer: (A)

The tests focus on how well the kidneys can concentrate urine and how ADH (vasopressin) status affects that concentration. In dogs with polyuria and polydipsia, measuring urine specific gravity and serum osmolality tells you whether the urine is being appropriately concentrated for the body's hydration state. If the urine is very dilute and serum osmolality is elevated, diabetes insipidus becomes likely, because the kidneys aren’t responding to or producing enough ADH. Psychogenic polydipsia, by contrast, often shows dilute urine due to excessive water intake but typically keeps serum osmolality closer to normal unless water intake is extreme.

If those initial tests don’t clearly separate conditions, a desmopressin (ADH analog) response test is used. In central diabetes insipidus, giving desmopressin will markedly increase urine concentration and reduce volume, because the problem is lack of ADH. In nephrogenic diabetes insipidus, the kidneys don’t respond to ADH, so little or no change occurs. Psychogenic polydipsia wouldn’t produce a DI pattern or a robust desmopressin response, helping distinguish it from DI.

So, start with urinary concentration (urine specific gravity) and serum osmolality; if still uncertain, use a desmopressin response test to separate central from nephrogenic DI and to differentiate DI from psychogenic polydipsia.