Osmolality, Blood and Urine

Osmolality, Blood and Urine is a topic covered in the Davis's Lab & Diagnostic Tests.

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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 UnitsSI Units (Conventional Units × 1)
Serum275–295 mOsm/kg275–295 mmol/kg
Urine (random)50–1200 mOsm/kg50–1200 mmol/kg
Urine (24-hr collection)
 Newborn75–300 mOsm/kg75–300 mmol/kg
 Children and adults250–900 mOsm/kg250–900 mmol/kg

Critical Findings and Potential Interventions

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 intravenous 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.

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