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Lab Manual for UCSF Clinical Laboratories

Lab Manual for SFGH

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OSMOLALITY, SERUM, PLASMA, URINE

Item Value
Available Stat? Yes
Test code Serum, Plasma: OSMO
Urine: UOSM
Performed by? Chemistry
Sendout? no
Price range Serum, Plasma: $
Urine: $$
In House Availability Routine: 7 days;
STAT: 24 hours/ 7 days
Principle Freezing point depression. Measures all volatile (ex: ethanol) and non-volatile solutes.
Interpretation 1. Serum hypo-osmolality, due to loss of sodium associated with diuretics, low salt diet, Addison's disease, adrenogenital syndrome, inappropriate secretion of ADH (trauma, tumor, etc.); excessive water replacement.

2. Serum hyper-osmolality, increased due to water restriction or loss, brain trauma with impaired release of ADH, chronic renal disease due to increased BUN, Diabetes mellitus due to increased glucose, diabetes insipidus.

3. Urine, during water restriction, the concentration should be more than 800 mOsm/kg. Minimal impairment, 600-800 mOsm/kg; moderate impairment, 400-600 mOsm/kg; severe impairment, less than 400 mOsm/kg. The urine osmolality may be impaired early, even before changes in other renal function tests, thus indicating early renal disease. Urine osmolality decreased in diabetes insipidus, increased with inappropriate secretion of ADH.
Container type Blood, gold or green top gel tube; Urine, sterile urine cup
Amount to Collect Blood, 3 mL; Urine, 15 mL
Sample type Serum, plasma, or urine
Min. Volume Blood, 2 mL; Urine, 1 mL
Normal range Serum/ Plasma Specimen - (mOsm/kg)
Premature Term Age 1-16 Adult
282-289 289-299 282-299 275-295


All values less than 240 or greater than 320 are considered Critical Values and are called to the physician or patient care unit immediately.

Urine Specimen – 50-1200 mOsm/kg (depending on solute and water intake)
Stability 24 hrs at room temperature, 24 hrs at 4°C, 6 months at -20°C
Interferences The difference between the calculated osmolality (see formula) and measured (if > 10 mOsm) suggests the presence of an exogenous substance, usually ethanol or some other low molecular weight alcohol.

Calculated mOsm/kg = 2[Na+] + glucose (mg/dL)/18 + BUN (mg/dL)/2.8
Additional information INDICATIONS:

1. Urine: During water restriction, urine osmolality is a sensitive test of decreased renal function: useful to R/O Diabetes mellitus, essential hypertension, silent pyelonephritis, and diabetes insipidus.

2. Serum: R/O presence of organic acids, sugars, or ethanol.

3. Urine mOsm/Serum mOsm Ratio: Diabetes insipidus (nephrogenic and neurogenic), ratio less than one; psychogenic polydipsia, ratio greater than one.
References 1. Henry, J.B. Clinical Diagnosis and Management by Laboratory Methods. 20th ed., Philadelphia: WB Saunders, p. 170-171, 2001.
CPT coding 83930, 83935
Last Updated 10/22/2012 7:24:51 PM
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