Urine specific gravity - the most underutilized test in veterinary medicine

Determining urine specific gravity is essential to localize azotemia, diagnose kidney failure (CKD), verify and localize the cause of polyuria, accurately interpret every urinalysis result, accurately interpret many serum biochemical values, ensure effective management of the acutely ill patient, and verify compliance of administration of several prescription foods and medications. Although urine specific gravity is a simple, easy, rapid, and inexpensive test to perform it is often overlooked or not requested until a time when its interpretation is confounded by treatment (i.e. after fluid therapy or medications that alter urine concentrating mechanisms) even though it is most useful when measured prior to therapy.

The kidneys play a fundamental role in tightly regulating plasma volume and composition to maintain osmolality despite fluctuations in fluid consumption, diet and disease. Regulation of water and solutes is the result of careful adjustments in glomerular filtration, tubular reabsorption, and tubular secretion. Measurement of urine osmolarity directly by osmometry or indirectly by urine specific gravity is the primary method used to evaluate the kidney’s response (i.e. concentration-removing water in excess of solute, or dilution-removing solute in excess of water) to the water needs of the body. Thus, evaluation of urine osmolarity or specific gravity is an index of tubular reabsorption of water in relation to solutes.

Increased thirst and urine production are common owner concerns. Knowledge of specific gravity is essential to verify polyuria and to differentiate its underlying causes (table 1). Another essential indication for routine evaluation of urine specific gravity involves interpretation of tests that are part of the complete urinalysis. Specific gravity is also used as an aid to monitor the patient’s fluid balance, especially during need for therapy with parenteral fluids. Most importantly urine specific gravity is used to localize azotemia and differentiate primary renal failure (table 2) from other polyuric disorders.

Glossary of common terms

Normal urine specific gravity

The specific gravity of normal animals is variable, being dependent on the fluid and electrolyte balance of the body, the protein, mineral and water composition of the diet, and other variables related to the species and individual. Urine specific gravity typically fluctuates widely from day to day and within the same day. A typical normal range for the dog is 1.001 to 1.060 in dogs and 1.001 to 1.080 in cats. Depending on the requirements of the body for water and/or other solutes, any specific gravity value within this range may be normal. Therefore, the concept of an average normal specific gravity is misleading because it implies that values below or above the average may be abnormal.

Concentrated urine

Urine is concentrated if it is significantly above the specific gravity of glomerular filtrate (greater than 1.008 to 1.012). The ability of patients to excrete concentrated urine is dependent on 1) a functional nervous system for production and release of antidiuretic hormone, 2) a sufficient number of nephrons to generate and maintain a high solute concentration in the renal medulla, and 3) a sufficient population of functioning tubules to respond to antidiuretic hormone. Data from partially nephrectomized dogs suggest that only about 1/3 of the nephrons of both kidneys is required to concentration urine to 1.030 or greater.

Diluted urine

Urine is dilute if it is significantly below the specific gravity of glomerular filtrate (less than 1.008 to 1.012). Metabolic work is required to dilute glomerular filtrate by removing solutes in excess of water. Therefore, a urine specific gravity below 1.008 indicates that a sufficient number of functioning nephrons (commonly estimated to be at least 1/3 of the total population, but urine dilution occurs even if functioning kidney mass is less than 1/3) are present to dilute urine, and therefore prevent the clinical signs associated with primary renal failure. Dilution is an appropriate and expected clinical response to over-hydration. However, formation of dilute urine when the patient is in negative water balance is abnormal and is usually an indication of the inability of antidiuretic hormone to stimulate receptors on distal renal tubules and collecting ducts (necessary to assemble water channels; examples include hypokalemia, hypercalcemia, urinary obstruction, nephrogenic diabetes insipidus), inability of the kidneys to generate a hyperosmotic medullary interstitium (recall that water only moves passively and needs a osmotic gradient; examples include kidney failure, hypoadrenocorticism, loop diuretics, decreased urea production), osmotic diuresis (diabetes mellitus and glucosuria, kidney failure and postobstructive diuresis), and lack of antidiuretic hormone (for example central diabetes insipidus).

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