Definition

Polyuria (PU) is defined as excessive urine production and is confirmed by demonstrating that daily urine production exceeds the upper limit of normal. In dogs, daily urine volume should normally not exceed approximately 50 mL/kg/day.¹ Normal daily urine volume is lower in cats. Frequency of voiding is not incl uded in the definition of PU and may be normal or increased in patients with PU. It is essential that PU not be confused with pollakiuria, the frequent voiding of small quantities of urine.

Polydipsia (PD) is consumption of water in excess of the upper limit of normal daily intake. Normal water intake in dogs is usually less than 60 mL/kg/day with an upper limit of 100 mL/kg/day.¹ Because water consumption is easier to measure, it is often used to confirm PD. However, environmental and behavioral factors (e.g., ambient temperature, activity of the animal) may markedly alter water consumption in dogs and should be considered in interpreting these measurements.² Normal values for water intake are likely substantially less in cats, but a well-defined cutoff value has not been established.

Diagnosis of PU or PD should ideally be confirmed by measuring water intake and/or urine production rate. Because water intake is more easily and accurately measured than urine output, confirmation of PU or PD is often made by measuring daily water intake. Although PD is usually a reliable surrogate for confirming PU or PD, increased water loss via nonurinary pathways (e.g., diarrhea) may be associated with PD absent PU.

Polyuric patients should have reduced urine specific gravity values; therefore, urine concentration is often used as a surrogate for confirming PU. Patients that persistently produce urine less concentrated than expected (i.e., <1.030 for dogs and <1.035 for cats) should be evaluated for a possible pathologic defect in urine-concentrating ability.³ In dogs, urine samples obtained in the morning immediately upon arising are typically the most concentrated and thus provide a more reliable estimate of urine concentrating ability.⁴ Urinalysis should be performed to determine urine concentration in any patient in which the owner reports increases in water intake or urination, regardless of whether the actual measured water intake exceeds the upper limit for PD. Patients with documented PD that have concentrated urine should be evaluated for nonurinary water losses.

Pathophysiology and Mechanism

Differential Diagnosis

Table 20-1 summarizes various recognized causes of PU and PD. Many of the causes of PU or PD present with other clinical signs or laboratory abnormalities, which readily facilitate their diagnosis (Table 20-2). In most instances, the cause for PU or PD can be determined from information gathered from history, physical examination, a complete blood count, serum chemistry profile, and urinalysis. The most common causes of PU and PD in dogs include renal disease, hyperadrenocorticism, and diabetes mellitus, whereas the most common causes of PU in cats include renal disease, hyperthyroidism, and diabetes mellitus. Gastrointestinal disease and gastrointestinal leiomyosarcoma have been linked to PU and PD.^7–9 Figure 20-1 is an algorithmic approach to diagnosis.

Table 20-1 Ruleouts for Polyuria and Polydipsia in Dogs and Cats

Table 20-2 Medical History and Physical Examination Findings Providing Clues to Possible Causes for Polyuria and Polydipsia

Figure 20-1 Algorithm demonstrating the diagnostic sequence recommended for establishing the diagnosis of PU and/or PD in dogs and cats. Tables 20-1 and 20-2 provide supplemental information useful in applying the algorithm to patients. ADH, Anti-diuretic hormone; CBC, complete blood count; DI, diabetes insipidus; GFR, glomerular filtration rate; T₄, thyroxine.