Diagnostic Approach to PU/PD: Urine Specific Gravity

Urinalysis is a major key in determining the presence of a water balance problem and the disorder causing the polyuria and polydipsia. The most important features of urinalysis are: the SG or osmolality; the presence or absence of glucose, protein or bacteria; and the cellularity of the sample.

A urine SG less than 1.030 in dogs and 1.035 in cats suggests a concentrating defect and supports the complaint of polyuria. Persistent glycosuria is diagnostic for primary renal glycosuria or, more commonly, diabetes mellitus. Significant proteinuria in the presence of an inactive urinary sediment and dilute urine can be associated with hyperadrenocorticism, pyelonephritis, pyometra, glomerulonephritis or other glomerulopathy.

An active urine sediment (pyuria, hematuria or bacteriuria) in a sample obtained by catheterization or cystocentesis supports urinary tract infection and possible pyelonephritis. Because urine sediment examination may be misleading in an extremely dilute urine sample, a urine culture should always be done to rule out pyelonephritis, regardless of sediment examination findings.

If the results of the above tests are unhelpful the direction of further diagnostic work-up can often be based on the urine SG (see Table below). For example, dogs and cats with a SG greater than 1.030--1.035 without glycosuria, are probably not polyuric and need no further work-up, at least for polyuria and polydipsia.

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Differential diagnosis based on urine specific gravity (SG) determination in animals with normal results of initial tests (CBC, serum biochemical profile and urinalysis).

Urine SG of 1.001--1.007

• Atypical hyperadrenocorticism (most common; always rule out first!)
  
• Atypical leptospirosis
  
• Psychogenic polydipsia
  
• Diabetes insipidus (complete)

Urine SG of 1.008--1.029

• Atypical hyperadrenocorticism (most common!)
  
• Atypical leptospirosis
  
• Early renal disease
  
• Typical and occult pyelonephritis
  
• Hyperthyroidism (cats)
  
• Psychogenic polydipsia
  
• Diabetes insipidus (partial)

Urine SG greater than 1.030 (without glycosuria)

• probably No further work-up for polyuria and polydipsia needed.

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Urine SG less than 1.008 A urine SG consistently less than 1.008 in a middle-aged to older dog is usually associated with diabetes insipidus, psychogenic polydipsia, atypical hyperadrenocorticism or atypical leptospirosis.

In these dogs with atypical hyperadrenocorticism, polyuria and polydipsia are major clinical signs but other characteristic clinical signs are mild or absent. In addition, these dogs with atypical disease may lack the serum biochemistry abnormalities commonly associated with hyperadrenocorticism (i.e. elevated serum alkaline phosphatase activity and hypercholesterolaemia). Results of adrenal function tests in these dogs are usually consistent with mild hyperadrenocorticism.

More recently an atypical form of leptospirosis has been recognized. These dogs present with an acute onset polyuria and polydipsia, hyposthenuria or isosthenuria, but no other laboratory abnormalities. Diagnosis of leptospira infection can be confirmed by positive leptospirosis serology or use of molecular detection of leptospiral DNA by polymerase chain reaction (PCR) testing performed on urine samples.

In general, when considering polyuric dogs with a urine SG less than 1.008, hyperadrenocorticism and atypical leptospirosis should be ruled out first before testing for central diabetes insipidus and primary polydipsia. There are several reasons for making this recommendation: the latter two disorders of water metabolism are much less common than hyperadrenocorticism (see Table below); the diagnostic tests of choice to differentiate these disorders – the water deprivation test or a therapeutic trial with the AVP-analogue desmopressin – are time-consuming and expensive. Also, dogs with hyperadrenocorticism may respond to these tests in a manner similar to dogs with central diabetes insipidus, resulting in a misdiagnosis. Moreover, water deprivation testing a dog with leptospirosis would be a major contraindication because of the possibility of causing significant patient morbidity.

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Differential rule outs for polyuria and polydipsia in dogs and cats, listed from most to least common. Dogs

• Hyperadrenocorticism
  
• Diabetes mellitus
  
• Chronic renal failure
  
• Pyelonephritis
  
• Pyometra
  
• Hypercalcaemia
  
• Atypical leptospirosis
  
• Psychogenic polydipsia
  
• Diabetes insipidus
  
• Liver disease
  
• Hypoadrenocorticism
  
• Acromegaly
  

Cats

• Chronic renal failure
  
• Diabetes mellitus
  
• Hyperthyroidism
  
• Hypercalcaemia
  
• Pyelonephritis
  
• Hypokalaemia
  
• Acromegaly
  
• Postobstructive diuresis
  
• Hyperadrenocorticism
  
• Hypoadrenocorticism
  
• Diabetes insipidus

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In cats, a urine SG consistently less than 1.008 is associated with either diabetes insipidus or hyperthyroidism. Obviously, hyperthyroidism should be ruled out first before initiating testing procedures for diabetes insipidus. It is also important to realize that the finding of a urine SG less than 1.008 in a cat or dog excludes mild (occult) renal disease, so precautions associated with the water deprivation test are not necessary.

Urine SG between 1.008 and 1.029

A urine SG of 1.008--1.012 or greater (but less than 1.030) can be associated with hyperadrenocorticism (dogs), hyperthyroidism (cats), or stage 1 renal insufficiency (including atypical leptospirosis) or pyelonephritis, as well as psychogenic polydipsia and partial forms of diabetes insipidus. 

Again, when considering animals with a urine SG greater than 1.008 hyperadrenocorticism and hyperthyroidism should first be ruled out. With this group of disorders, pyelonephritis and early renal insufficiency should next be ruled out before evaluating the animal for psychogenic polydipsia and diabetes insipidus with a water deprivation test. Performing a water deprivation test as a diagnostic tool in the face of unsuspected renal insufficiency or pyelonephritis could induce overt renal failure or urosepsis. To avoid this complication, a sensible approach is to do the following:

1. Perform a urine culture to help exclude pyelonephritis and associated urinary tract infection.
   
2. Consider leptospirosis serology and urine PCR testing.
   
3. Evaluate renal size and architecture by abdominal radiography or, preferably, renal ultrasonography. The ultrasonographic appearance of renal parenchymal disease (chronic renal failure) includes increased cortical echogenicity and loss of a distinct corticomedullary junction. The kidneys may appear smaller than normal and have an ill-defined or irregular border. Similar sonographic findings, in addition to a dilated renal pelvis, are characteristic of pyelonephritis.

If urine culture results are negative, leptopirosis serology and urine PCR testing are negative, and radiographic or ultrasonographic findings are equivocal, a creatinine or iohexol clearance test or renal biopsy may be indicated. In rare cases, the urine culture may be negative even if pyelonephritis is present. If clinical or ultrasonographic findings suggest occult pyelonephritis, a therapeutic trial with an appropriate antibiotic (e.g. enrofloxacin) should be instituted.

In the next post, I will talk about when water deprivation testing is needed in the workup of dogs and cats with PU/PD.