Osmolality (Urine)

Synonym

Tubes

5-10 mL midstream random urine sample in a disposable sterile plastic collection container or plain tube
OR
Timed urine specimen of 12-hours or 24-hours urine sample collected in a special container (no preservatives added)

Additional information

Use a freshly voided specimen for midstream random urine sample
Send specimen to lab immediately and if delayed >1hr refrigerate the specimen
Skin epithelium, bacteria, secretions, hair, lint, etc. should not contaminate the sample
Instructions for clean catch midstream urine collection:

Thoroughly wash her hands
Cleanse the labia from front to back
While keeping the labia separated, void a small amount into the toilet
Without interrupting the urine stream, void directly into the specimen container

Patient empties the bladder. This urine is discarded
Then, all urine is collected into a container for the next 12/24 hours
At 12/24 hr since the start of urine collection, bladder is emptied and this urine is added to the container
Stored in refrigerator or container kept on ice and taken out only to add more urine
Note the time and date on collection container

Info

The urine osmolality test is an indirect measurement of the concentration of particles that contribute to osmolality in urine
Urine osmolality is contributed by sodium, potassium, bicarbonate, chloride, glucose, and urea, and is regulated by anti-diuretic hormone (ADH)
Urine osmolality can be estimated by two methods:

Direct measurement by using freezing point depression osmometer
Indirect measurement by calculation of estimated values of sodium, potassium, urea, and glucose by the following formula:
Calculated urine osmolality = 2(Na + K) + urea + glucose

Clinical

Evaluation of the renal tubular concentrating-ability of the kidneys
Aids in the detection and diagnosis of renal diseases, diabetes insipidus (DI), and syndrome of inappropriate antidiuretic hormone secretion (SIADH)
Evaluation of electrolyte and acid-base balance
Aids in the differential diagnosis in patients with hypernatremia or hyponatremia, and assists in distinguishing between primary polydipsia and other causes of hyponatremia
Assists in distinguishing between primary polydipsia and central or nephrogenic diabetes insipidus (DI) as a cause of polyuria
Aids in the calculation of urine osmolal gap and characterization of metabolic acidosis and renal tubular acidosis (along with ammonia excretion)
Assists in the diagnosis of metabolic, renal, and endocrine (ADH) disorders
To determine whether water excretion is normal or impaired in the hypoosmolar state (serum osmolality <280 mOsm/kg H₂O)

Elevated urine osmolality is consistent with pre-renal impairment (>500 mOsm/kg H₂O), whereas urine osmolality is similar to plasma osmolality in acute tubular necrosis (ATN/ renal kidney failure)
SIADH is suspected when urine osmolality is >200 mOsm/kg H₂O along with the presence of urine sodium >20 mOsm/kg H₂O, low serum sodium, patient is not dehydrated, and renal, adrenal, thyroid, cardiac, or liver disease, and drug interference is ruled out
Low birth weight infants tend to have increased serum osmolality with normal urine osmolality
Complete and appropriate suppression of antidiuretic hormone secretion is indicated by urine osmolality of <100 mOsm/kg H₂O
A low urine osmolal gap (< 100 mOsm/kg H₂O) suggests low ammonium excretion and thus renal tubular acidosis (RTA)

Additional information

Urine osmolality must be interpreted in the light of appropriate physiological response to the state of hydration of the patient
The simultaneous determination of serum and urine osmolality, as in urinary/serum ratio, is helpful to compare values between the two fluids. Normal ranges for the urinary/serum ratio are 0.2–4.7 and may be >3 with overnight dehydration
Factors interfering with the test results include:

Recent administration of radiographic contrast medium
Recent blood transfusion
Mannitol, sorbitol, or glycine solutions used as irrigation during transurethral resection of the prostate

Nl Result

Consult your laboratory for their normal ranges as these may vary somewhat from the ones listed below.

Urine/Serum Osmolality

High Result

Conditions associated with elevated levels of urine osmolality include:

Low Result

Conditions associated with decreased levels of urine osmolality includes:

References

ARUP Laboratories®. Osmolality, Urine. [Homepage on the internet]©2007. Last accessed on July 31, 2007. Available at URL: http://www.aruplab.com/guides/ug/tests/0020228.jsp
eMedicine from WebMD®. Hyponatremia. [Homepage on the Internet] ©1996-2007. Last updated on January 18, 2007. Last accessed on July 31, 2007. Available at URL: http://www.emedicine.com/emerg/topic275.htm
KamelKS et al. A new classification for renal defects in net acid excretion. Am J Kirdney Dis. 1997 Jan;29(1):136-46.
Kugler JP et al. Hyponatremia and hypernatremia in the elderly. Am Fam Physician 2000 Jun 15;61(12):3623-30. Available at URL: http://www.aafp.org/afp/20000615/3623.html
Laboratory Corporation of America®. Osmolality, Urine. [Homepage on the internet]©2007. Last accessed on July 31, 2007. Available at URL: http://www.labcorp.com/datasets/labcorp/html/chapter/mono/sc017000.htm
Lab Tests online®. Osmolality. [Homepage on the Internet]©2001-2007. Last reviewed on January 30, 2006. Last accessed on July 31, 2007. Available at URL: http://www.labtestsonline.org/understanding/analytes/osmolality/glance.html
Samarasinghe S et al. Diabetes insipidus. Expert Rev Anticancer Ther. 2006 Sep;6 Suppl 9:S63-74.
Wilting I et al. Urine osmolality, cyclic AMP and aquaporin-2 in urine of patients under lithium treatment in response to water loading followed by vasopressin administration. Eur J Pharmacol. 2007 Jul 2;566(1-3):50-7. Epub 2007 Mar 30.