Test Details
Methodology
Freezing point depression
Result Turnaround Time
1 - 3 days
Turnaround time is defined as the usual number of days from the date of pickup of a specimen for testing to when the result is released to the ordering provider. In some cases, additional time should be allowed for additional confirmatory or additional reflex tests. Testing schedules may vary.
Use
Evaluate concentrating ability of the kidneys (eg, in acute and chronic renal failure); evaluate electrolyte and water balance; used in work-up for renal disease, syndrome of inappropriate antidiuretic hormone secretion (SIADH), and diabetes insipidus; may be used with urinalysis when patient has had radiopaque substances, has glycosuria or proteinuria;1 evaluate dehydration, amyloidosis. Osmolality is desirable in examination of neonatal urine when protein or glucose are present.2
Special Instructions
If 24-hour urine is collected, record total urine volume on the request form.
Limitations
This test has not been validated for the measurement of osmolality of fetal urine.
Footnotes
Custom Additional Information
Osmolality is a better measurement than specific gravity. Osmolality is a measure of renal tubular concentration, depending on the state of hydration.
Simultaneous determination of urine and serum osmolalities facilitates interpretation of results.
High urinary:plasma ratio is seen in concentrated urine. Normal ranges for the U:P ratio are given by Weisberg as approximately 0.2−4.7, and >3.0 with overnight dehydration.1 With poor concentrating ability the ratio is low but still ≥1.0. In SIADH urine sodium and urine osmolality are high for plasma osmolality.3
Low birthweight infants have been reported to have increased serum osmolality with normal urine osmolality.4
The urine osmolar gap is described as the sum of urinary concentrations of sodium, potassium, bicarbonate, chloride, glucose, and urea compared to measured urine osmolality. The gap is normally 80−100 mOsm/kg (SI: 80−100 mmol/kg) H2O. Determination of the urine osmolal gap is used to characterize metabolic acidosis.
Specimen Requirements
Specimen
Urine (random or 24-hour)
Volume
2 mL
Minimum Volume
0.2 mL (Note: This volume does not allow for repeat testing.)
Container
Plastic urine container, no preservative
Collection Instructions
Centrifuge urine sample after collection for five minutes at 4°C to 25°C to remove gross particulate matter.
Stability Requirements
| Temperature | Period |
|---|---|
| Room temperature | 14 days |
| Refrigerated | 14 days |
| Frozen | 14 days |
| Freeze/thaw cycles | Stable x3 |
Reference Range
• 24-hour urine: 300−900 mOsm/kg
• Random urine: 50−1400 mOsm/kg
• After 12-hour fluid restriction: >850 mOsm/kg
Storage Instructions
Refrigerate.
Causes for Rejection
Fetal urine sample received
LOINC® Map
| Order Code | Order Code Name | Order Loinc | Result Code | Result Code Name | UofM | Result LOINC |
|---|---|---|---|---|---|---|
| 003442 | Osmolality, Urine | 2695-5 | 003442 | Osmolality, Urine | mOsmol/kg | 2695-5 |
| Order Code | 003442 | |||||
| Order Code Name | Osmolality, Urine | |||||
| Order Loinc | 2695-5 | |||||
| Result Code | 003442 | |||||
| Result Code Name | Osmolality, Urine | |||||
| UofM | mOsmol/kg | |||||
| Result LOINC | 2695-5 |