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Test ID UOSMU Osmolality, Random, Urine

Reporting Name

Osmolality, U

Useful For

Assessing the concentrating and diluting ability of the kidney


Specimen Required


Supplies: Urine Tubes, 10 mL (T068)

Container/Tube: Plastic, 10-mL urine tube

Specimen Volume: 5 mL

Collection Instructions: Collect a random urine specimen


Specimen Minimum Volume

1 mL

Reference Values

0-11 months: 50-750 mOsm/kg

≥12 months: 150-1,150 mOsm/kg

Test Classification

This test has been cleared, approved, or is exempt by the US Food and Drug Administration and is used per manufacturer's instructions. Performance characteristics were verified by Mayo Clinic in a manner consistent with CLIA requirements.

CPT Code Information

83935

Clinical Information

Osmolality is an index of the solute concentration. Urine osmolality is a measure of the concentration of osmotically active particles, principally sodium, chloride, potassium, and urea; glucose can contribute significantly to the osmolality when present in substantial amounts in urine. Urinary osmolality corresponds to urine specific gravity in nondisease states.

 

The ability of the kidney to maintain both tonicity and water balance of the extracellular fluid can be evaluated by measuring the osmolality of the urine either routinely or under artificial conditions. More information concerning the state of renal water handling or abnormalities of urine dilution or concentration can be obtained if urinary osmolality is compared to serum osmolality and if urine electrolyte studies are performed. Normally, the ratio of urine osmolality to serum osmolality is 1.0 to 3.0, reflecting a wide range of urine osmolality.

Interpretation

With normal fluid intake and normal diet, a patient will produce urine of about 500 to 850 mosmol/kg water. Above age of 20 years, there is an age dependent decline in the upper reference range of approximately 5 mOsm/kg/year.

 

The normal kidney can concentrate urine to 800 to 1400 mosmol/kg and with excess fluid intake, a minimal osmolality of 40 to 80 mosmol/kg can be reached.

 

With dehydration, the urine osmolality should be 3 to 4 times the plasma osmolality.

Cautions

No significant cautionary statements

Clinical Reference

1. Newman D, Price C: Renal function and nitrogen metabolites. In: Burtis CA, Ashwood ER, eds. Tietz Textbook of Clinical Chemistry. 4th ed. WB Saunders Company; 2006

2. Delaney MP, Lamb EJ: Kidney disease. In: Rifai N, Horvath AR, Wittwer CT, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018:1306

Method Description

Measurement of the freezing point of urine is the most widely used principle in osmometers. The extent of lowering below 0° C (the freezing point of water) is a function of the concentration of substances dissolved in the urine. By definition 1 milliosmole per kilogram lowers the freezing point 0.001858° C.(Schindler EI, Brown SM, Scott MG: Electrolytes and blood gases. In: Rifai N, Horvath AR, Wittwer CT, eds: Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed.2018:610-612)

Reject Due To

  All specimens will be evaluated at Mayo Clinic Laboratories for test suitability.

NY State Approved

Yes

Method Name

Freezing Point Depression

Specimen Type

Urine

Specimen Stability Information

Specimen Type Temperature Time Special Container
Urine Refrigerated (preferred) 7 days
  Frozen  7 days

Forms

If not ordering electronically, complete, print, and send a Renal Diagnostics Test Request (T830) with the specimen.

Day(s) Performed

Monday through Sunday

Report Available

Same day/1 day