Synonym/Acronym
osmo.
Rationale
To assess fluid and electrolyte balance related to hydration, acid-base balance, and screening for toxins.
Patient Preparation
There are no food, fluid, activity, or medication restrictions unless by medical direction. As appropriate, provide the required urine collection container and specimen collection instructions.
Normal Findings
Method: Freezing point depression.
| Conventional Units | SI Units (Conventional Units × 1) |
|---|
| Serum | 275–295 mOsm/kg | 275–295 mmol/kg |
| Urine (random) | 50–1,200 mOsm/kg | 50–1,200 mmol/kg |
| Urine (24-hr collection) | | |
| Newborn | 75–300 mOsm/kg | 75–300 mmol/kg |
| Children and adults | 250–900 mOsm/kg | 250–900 mmol/kg |
Serum
- Less than 265 mOsm/kg (SI: Less than 265 mmol/kg)
- Greater than 320 mOsm/kg (SI: Greater than 320 mmol/kg)
Timely notification to the requesting health-care provider (HCP) of any critical findings and related symptoms is a role expectation of the professional nurse. A listing of these findings varies among facilities.
Serious clinical conditions may be associated with elevated or decreased serum osmolality. The following conditions are associated with elevated serum osmolality:
- Respiratory arrest: 360 mOsm/kg (SI: 360 mmol/kg)
- Stupor of hyperglycemia: 385 mOsm/kg (SI: 385 mmol/kg)
- Grand mal seizures: 420 mOsm/kg (SI: 420 mmol/kg)
- Death: Greater than 420 mOsm/kg (SI: Greater than 420 mmol/kg)
Symptoms of critically high levels include poor skin turgor, listlessness, acidosis (decreased pH), shock, seizures, coma, and cardiopulmonary arrest. Intervention may include close monitoring of electrolytes, administering IV fluids with the appropriate composition to shift water either into or out of the intravascular space as needed, monitoring cardiac signs, continuing neurological checks, and taking seizure precautions.
(Study type: Blood collected in a gold-, red-, or red/gray-top tube; urine from an unpreserved random specimen collected in a clean plastic collection container; related body system: Endocrine and urinary systems.)
Osmolality is a measure of the number of particles in a solution; it is independent of particle size, shape, and charge (unlike specific gravity). Osmolality is used to assist in the diagnosis of metabolic, renal, and endocrine disorders. Measurement of osmotic concentration in serum provides clinically useful information about water and dissolved-particle transport across fluid compartment membranes. Urine osmolality provides information about the ability of the kidneys to concentrate urine. Urine osmolality, like serum osmolality, can be used to evaluate, monitor, and treat imbalances in fluid and electrolyte concentrations. The simultaneous determination of serum and urine osmolality provides the opportunity to compare values between the two fluids and get a better understanding of the issues involved. A normal urine-to-serum ratio is approximately 0.2 to 4.7 for random samples and greater than 3 for first-morning samples (dehydration normally occurs overnight). The adrenal glands and kidneys are important organs in maintaining electrolyte and water balance through complex feedback loops involving secretion or suppression of antidiuretic hormone (ADH) and aldosterone by the adrenals and angiotensin and renin by the kidneys. Abnormalities that affect ADH levels will also affect osmolality requiring a differential diagnosis in order to provide effective, condition-specific treatment. For additional information regarding fluid and electrolyte balance, refer to the studies titled “Aldosterone,” “Antidiuretic Hormone,” and “Renin.”
Sodium levels have a significant effect on osmolality; in general, the same conditions that increase or decrease serum sodium levels have the same effect on osmolality. The major dissolved particles that contribute to osmolality are sodium, chloride, bicarbonate, urea, and glucose. Some of these substances are used in the following calculated estimate: 
Serum/urine osmolality = (2 × Na+) + (glucose/18) + (BUN/2.8)
Measured osmolality in serum or urine is higher than the estimated value because of other unmeasured organic particles in solution contributing to the osmotic concentration. The osmolal gap or delta gap is the difference between the measured and calculated values and is normally 5 to 10 mOsm/kg. If the difference is greater than 15 mOsm/kg, consider ethylene glycol, isopropanol, methanol, or ethanol toxicity. These substances behave like antifreeze, lowering the freezing point in the blood, and provide misleadingly high results.
Before the Study: Planning and Implementation
Teaching the Patient What to Expect
- Discuss how this test is used to evaluate electrolyte and water balance.
- Explain that a blood or urine sample is needed for the test. Information regarding urine specimen collection is presented with other general guidelines in Appendix A: Patient Preparation Specimen Collection.
- Either a random or a timed urine collection may be requested.
Potential Nursing Actions
- Include on the timed collection container’s label urine total volume, test start and stop times/dates, and any medications that may interfere with test results.
After the Study: Implementation & Evaluation Potential Nursing Actions
Treatment Considerations
- Discuss how increased osmolality may be associated with dehydration.
- Dehydration is a significant and common finding in older adults and other patients in whom renal function has deteriorated. Discuss the symptoms of adult dehydration: confusion, dizziness, excessive thirst, fatigue, dark urine, infrequent urination. Discuss the symptoms of pediatric dehydration: sunken eyes, sunken cheeks, sunken soft spot, dry diapers for 3 hr, crying without tears, dry mouth, dry tongue, listlessness, irritability.
Nutritional Considerations
- Decreased osmolality may be associated with overhydration. Observe for signs and symptoms of fluid-volume excess related to excess electrolyte intake, fluid-volume deficit related to active body fluid loss, or risk of injury related to an alteration in body chemistry. (For electrolyte-specific dietary references, see blood studies titled “Chloride, Blood” “Potassium, Blood and Urine,” and “Sodium, Blood and Urine.”)
Clinical Judgement
- Consider how to emphasize the necessity of adequate hydration to overall health.
Follow-Up Evaluation and Desired Outcomes
- Acknowledges contact information provided for the National Kidney Foundation (www.kidney.org).