Polyuria and polydipsia. Problems associated with patient evaluation

Primary disorders of water balance (central diabetes insipidus, congenital nephrogenic diabetes insipidus, and psychogenic polydipsia) should always be considered in the differential diagnosis of polyuria and polydipsia. In general, animals with these disorders have only one laboratory abnormality, a low urine specific gravity. The more common causes of polyuria and polydipsia (eg, hypercalcemia, chronic renal insufficiency, pyelonephritis, hyperadrenocorticism), in most instances, have specific and obvious abnormalities associated with the complete blood count (CBC), serum chemistry profile, and urinalysis. However, in some cases, a low urine specific gravity may initially be the only abnormality in these more common ruleouts. The workup for polyuria and polydipsia, especially in those cases with normal or near normal blood work, can be tedious, time consuming, confusing, and not without significant patient morbidity. This chapter will focus on the problems associated with diagnostic testing used to evaluate animals with disorders of water balance.