Type of test Urine

 Normal findings

 12- to 14-hour fluid restriction: > 850 mOsm/kg H2 O (SI units)

Random specimen: 50-1200 mOsm/kg H2 O, depending on fluid intake, or 50-1200 mmol/kg (SI units)

Test explanation and related physiology

Osmolality is the measurement of the number of dissolved particles in a solution. It is a more exact measurement of urine concentration than specific gravity.

Osmolality is used in the precise evaluation of the concentrating and diluting abilities of the kidney. With normal fluid intake and normal diet, a patient will produce a urine of about 500 to 850 mOsm/kg water. The normal kidney can concentrate a urine to 800 to 1400 mOsm/kg. With excess fluid intake, a minimal osmolality of 40 to 80 mOsm/kg can be obtained. With dehydration, the urine osmolality should be three to four times the plasma osmolality.

Osmolality is used in the evaluation of kidney function and the ability to excrete ammonium salts. Osmolality may be used as part of the urinalysis when the patient has glycosuria or proteinuria or has had tests that use radiopaque substances. In these situations, the urine osmolar gap increases because of other organic osmolar particles. The urine osmolar gap is the sum of all the particles predicted or calculated to be in the urine (electrolytes, urea, and glucose) compared with the actual measurement of the osmolality. The predicted or calculated urine osmolality can then be determined by urine levels of sodium, potassium, glucose, and urea nitrogen.

Normally, the osmolar gap is 80 to 100 mOsm/kg of H₂O. The urine osmolality is more easily interpreted when the serum osmolality (see previous study) is simultaneously performed. More information concerning the state of renal water handling or abnormalities of urine dilution or concentration can be obtained if urinary osmolality is compared with serum osmolality and if urine electrolyte studies are performed. Normally, the ratio of urine osmolality to serum osmolality is 1 to 3, reflecting a wide range of urine osmolality.

Procedure and patient care

 • See inside front cover for Routine Urine Testing.

* Tell the patient that no special preparation is necessary for a random urine specimen.

* Inform the patient that preparation for a fasting urine specimen may require a high-protein diet for 3 days before the test.

* Instruct the patient to eat a dry supper the evening before the test and to drink no fluids until the test is completed the next morning.

 • Preferably, collect a first-voided urine specimen for a random sample.

• Indicate on the laboratory slip the patient’s fasting status.

 Abnormal findings

 Increased levels

- SIADH secretion

- Acidosis

- Shock

- Hypernatremia

- Hepatic cirrhosis

- Congestive heart failure

- Addison disease

Decreased levels

- Diabetes insipidus

- Hypercalcemia

- Excess fluid intake

- Renal tubular necrosis

- Aldosteronism

- Hypokalemia

- Severe pyelonephritis

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