Osmolality, Blood and Urine

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Osmolality, Blood and Urine

Synonym/acronym: Osmo.

Common use

To assess fluid and electrolyte balance related to hydration, acid-base balance, and toxic screening; to assist in diagnosing diseases such as diabetes.


Serum (1 mL) collected in a gold-, red-, or red/gray-top tube; urine (5 mL) from an unpreserved random specimen collected in a clean plastic collection container.

Normal findings

(Method: Freezing point depression)
Conventional UnitsSI Units (Conventional Units × 1)
Serum275–295 mOsm/kg275–295 mmol/kg
 Newborn75–300 mOsm/kg75–300 mmol/kg
 Children and adults250–900 mOsm/kg250–900 mmol/kg


Osmolality is the number of particles in a solution; it is independent of particle size, shape, and charge. Measurement of osmotic concentration in serum provides clinically useful information about water and dissolved-particle transport across fluid compartment membranes. Osmolality is used to assist in the diagnosis of metabolic, renal, and endocrine disorders. The simultaneous determination of serum and urine osmolality provides the opportunity to compare values between the two fluids. A normal urine-to-serum ratio is approximately 0.2 to 4.7 for random samples and greater than 3.0 for first-morning samples (dehydration normally occurs overnight). 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 osmolality = (2 × Na+) + (glucose/18) + (BUN/2.8)

Measured osmolality is higher than the estimated value. The osmolal 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.

This procedure is contraindicated for




  • Assist in the evaluation of antidiuretic hormone (ADH) function
  • Assist in rapid screening for toxic substances, such as ethylene glycol, ethanol, isopropanol, and methanol
  • Evaluate electrolyte and acid-base balance
  • Evaluate state of hydration
  • Urine

  • Evaluate concentrating ability of the kidneys
  • Evaluate diabetes insipidus
  • Evaluate neonatal patients with protein or glucose in the urine
  • Perform work-up for renal disease

Potential diagnosis

Increased in

  • Serum
    • Azotemia (related to accumulation of nitrogen-containing waste products that contribute to osmolality)
    • Dehydration (related to hemoconcentration)
    • Diabetes insipidus (related to excessive loss of water through urination that results in hemoconcentration)
    • Diabetic ketoacidosis (related to excessive loss of water through urination that results in hemoconcentration)
    • Hypercalcemia (related to electrolyte imbalance that results in water loss and hemoconcentration)
    • Hypernatremia (related to insufficient intake of water or excessive loss of water; sodium is a major cation in the determination of osmolality)
  • Urine
    • Amyloidosis
    • Azotemia (related to decrease in renal blood flow; decrease in water excreted by the kidneys results in a more concentrated urine)
    • Congestive heart failure (decrease in renal blood flow related to diminished cardiac output; decrease in water excreted by the kidneys results in a more concentrated urine)
    • Dehydration (related to decrease in water excreted by the kidneys that results in a more concentrated urine)
    • Hyponatremia
    • Syndrome of inappropriate antidiuretic hormone production (SIADH) (related to decrease in water excreted by the kidneys that results in a more concentrated urine)

Decreased in

    • Adrenocorticoid insufficiency
    • Hyponatremia (sodium is a major influence on osmolality; decreased sodium contributes to decreased osmolality)
    • SIADH (related to increase in water reabsorbed by the kidneys that results in a more dilute serum)
    • Water intoxication (related to excessive water intake, which has a dilutional effect)
    • Diabetes insipidus (related to decreased ability of the kidneys to concentrate urine)
    • Hypernatremia (related to increased water excreted by the kidneys that results in a more dilute urine)
    • Hypokalemia (related to increased water excreted by the kidneys that results in a more dilute urine)
    • Primary polydipsia (related to increase in water intake that results in dilute urine)

Critical findings


  • Less than 265 mOsm/kg (SI: Less than 265 mmol/kg)
  • Greater than 320 mOsm/kg (Greater than 320 mmol/kg)
  • Note and immediately report to the health-care provider (HCP) any critically increased or decreased values and related symptoms.

  • It is essential that a critical finding be communicated immediately to the requesting health-care provider (HCP). A listing of these findings varies among facilities.

  • Timely notification of a critical finding for lab or diagnostic studies is a role expectation of the professional nurse. Notification processes will vary among facilities. Upon receipt of the critical value the information should be read back to the caller to verify accuracy. Most policies require immediate notification of the primary HCP, Hospitalist, or on-call HCP. Reported information includes the patient’s name, unique identifiers, critical value, name of the person giving the report, and name of the person receiving the report. Documentation of notification should be made in the medical record with the name of the HCP notified, time and date of notification, and any orders received. Any delay in a timely report of a critical finding may require completion of a notification form with review by Risk Management.

  • 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
  • Stupor of hyperglycemia: 385 mOsm/kg
  • Grand mal seizures: 420 mOsm/kg
  • Death: greater than 420 mOsm/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.

Interfering factors

  • Drugs that may increase serum osmolality include corticosteroids, glycerin, inulin, ioxitalamic acid, mannitol, and methoxyflurane.
  • Drugs that may decrease serum osmolality include bendroflumethiazide, carbamazepine, chlorpromazine, chlorthalidone, cyclophosphamide, cyclothiazide, doxepin, hydrochlorothiazide, lorcainide, methyclothiazide, and polythiazide.
  • Drugs that may increase urine osmolality include anesthetic agents, chlorpropamide, cyclophosphamide, furosemide, mannitol, metolazone, octreotide, phloridzin, and vincristine.
  • Drugs that may decrease urine osmolality include captopril, demeclocycline, glyburide, lithium, methoxyflurane, octreotide, tolazamide, and verapamil.

Nursing Implications and Procedure


  • Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
  • Patient Teaching: Inform the patient that the test is used to evaluate electrolyte and water balance.
  • Obtain a history of the patient’s complaints, including a list of known allergens, especially allergies or sensitivities to latex.
  • Obtain a history of the patient’s endocrine and genitourinary systems, symptoms, and results of previously performed laboratory tests and diagnostic and surgical procedures.
  • Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values online at DavisPlus).
  • Review the procedure with the patient. Inform the patient that blood specimen collection takes approximately 5 to 10 min; random urine collection takes approximately 5 min and depends on the cooperation of the patient. Urine specimen collection may also be timed. Address concerns about pain and explain that there may be some discomfort during the venipuncture; there will be no discomfort during urine collection.
  • Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
  • Note that there are no food, fluid, or medication restrictions unless by medical direction.


  • Potential complications: N/A
  • Direct the patient to breathe normally and to avoid unnecessary movement during the venipuncture.
  • Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient, and label the appropriate specimen container with the corresponding patient demographics, initials of the person collecting the specimen, date, and time of collection. Perform a venipuncture as appropriate.
  • Blood

  • Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
  • Perform a venipuncture.
  • Remove the needle and apply direct pressure with dry gauze to stop bleeding. Observe/assess venipuncture site for bleeding or hematoma formation and secure gauze with adhesive bandage.
  • Urine

  • Provide a nonmetallic urinal, bedpan, or toilet-mounted collection device.
  • Either a random specimen or a timed collection may be requested. For timed specimens, a 12- or 24-hr time frame for urine collection may be ordered. Inform the patient that all urine must be saved during that 12- or 24-hr period. Instruct the patient not to void directly into the laboratory collection container. Instruct the patient to avoid defecating in the collection device and to keep toilet tissue out of the collection device to prevent contamination of the specimen. Place a sign in the bathroom to remind the patient to save all urine.
  • Instruct the patient to void all urine into the collection device and then to pour the urine into the laboratory collection container. Alternatively, the specimen can be left in the collection device for a health-care staff member to add to the laboratory collection container.
  • Clean-Catch Specimen

  • Instruct the male patient to (1) thoroughly wash his hands, (2) cleanse the meatus, (3) void a small amount into the toilet, and (4) void directly into the specimen container.
  • Instruct the female patient to (1) thoroughly wash her hands; (2) cleanse the labia from front to back; (3) while keeping the labia separated, void a small amount into the toilet; and (4) without interrupting the urine stream, void directly into the specimen container.
  • Indwelling Catheter

  • Put on gloves. Empty drainage tube of urine. It may be necessary to clamp off the catheter for 15 to 30 min before specimen collection. Cleanse specimen port with antiseptic swab, and then aspirate 5 mL of urine with a 21- to 25-gauge needle and syringe. Transfer urine to a sterile container.
  • Blood or Urine

  • Promptly transport the specimen to the laboratory for processing and analysis.


  • Inform the patient that a report of the results will be made available to the requesting HCP, who will discuss the results with the patient.
  • Nutritional Considerations: Decreased osmolality may be associated with overhydration. Observe the patient 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 monographs titled “Chloride,” “Potassium,” and “Sodium.”)
  • Increased osmolality may be associated with dehydration. Evaluate the patient for signs and symptoms of dehydration. Dehydration is a significant and common finding in geriatric and other patients in whom renal function has deteriorated.
  • Recognize anxiety related to test results. Discuss the implications of abnormal test results on the patient’s lifestyle. Provide teaching and information regarding the clinical implications of the test results, as appropriate. Educate the patient regarding access to counseling services. Provide contact information, if desired, for the National Kidney Foundation (www.kidney.org).
  • Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Answer any questions or address any concerns voiced by the patient or family.
  • Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

Related Monographs

  • Related tests include ACTH, anion gap, ammonia, ADH, ANP, BNP, BUN, calcium, carbon dioxide, chloride, CBC hematocrit, CBC hemoglobin, cortisol, creatinine, echocardiography, echocardiography transesophageal, ethanol, glucose, ketones, lung perfusion scan, magnesium, phosphorus, potassium, sodium, and UA.
  • Refer to the Endocrine and Genitourinary systems tables at the end of the book for related tests by body system.
Handbook of Laboratory and Diagnostic Tests, © 2013 Farlex and Partners
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