Patient Test Information


Formal name:

Osmolality (plasma, urine, stool)

Related tests:

BUN, Creatinine, Electrolytes, Glucose, Sodium, Emergency and Overdose Drug Testing, ADH

Why Get Tested?

To help evaluate the body's water and electrolyte balance; to investigate low sodium levels in the blood (hyponatremia) and increased or decreased urine production; to detect alcohol poisoning due to ingestion of toxins such as methanol or ethylene glycol; to monitor the effectiveness of treatment for conditions affecting osmolality; to help determine the cause of chronic diarrhea

When to Get Tested?

When someone has a low blood sodium level, is taking mannitol (an osmotic diuretic), may have ingested methanol, ethylene glycol, or isopropyl alcohol, is producing significantly increased or decreased amounts of urine, or has chronic diarrhea

Sample Required?

A blood sample drawn from a vein in your arm and/or a random urine sample; sometimes a fresh, liquid stool sample

Test Preparation Needed?

None may be required or you may be instructed to fast for 6 hours before the test; follow any instructions provided. Inform your health care provider of all medications you are taking, especially mannitol.

How is it used?

The osmolality test provides a snapshot of the number of solutes present in the blood (serum), urine, or stool. It is ordered to help evaluate the body's water balance or its ability to produce and concentrate urine, to help investigate low sodium levels (hyponatremia), to detect the presence of toxins such as methanol and ethylene glycol, and to monitor osmotically active drug therapies such as mannitol, used to treat cerebral edema. It is also ordered to help monitor the effectiveness of treatment for any conditions found to be affecting a person's osmolality.

Serum osmolality is primarily ordered to investigate hyponatremia. Hyponatremia may be due to sodium loss through the urine or to increased fluid in the bloodstream. Increased fluid may be due to drinking excessive amounts of water, water retention, decreased ability of the kidneys to produce urine, or the presence of osmotically active agents such as glucose, mannitol, or glycine (a chemical used in surgical irrigation fluids). Marathon runners, for example, can experience acute hyponatremia by drinking large quantities of water in a short period of time to replace fluid lost in sweat. People who chronically drink excessive amounts of water, by choice or due to a psychological condition, may have chronic hyponatremia. Someone may also appear to have low sodium when the percentage of water in their blood decreases due to the presence of increased proteins or lipids.

Mannitol, glycine, and the ingestion of toxins such as methanol, ethylene glycol, isopropyl alcohol, and propylene glycol can be detected, evaluated, and monitored by ordering an osmotic gap (also called osmolal gap). This calculation compares measured osmolality with measurements of the major solutes. The osmotic gap is the difference between them. An osmolal gap of greater than 10 is considered abnormal and represents the presence of an osmotically active substance in the blood.

In order to calculate the osmotic gap, tests for blood sodium, blood urea nitrogen (BUN), and glucose must be performed to calculate the expected osmolality. Some versions of the expected osmolality calculation also include the measurement of ethanol. An example calculation is:

Serum Osmolality Calculation (ethanol not always included)

2 x (Na+) + (Glucose/18) + (BUN/2.8) + (Ethanol/3.8)

Note: Glucose, BUN, and ethanol may be reported in mg/dL (milligrams per deciliter) or mmol/L (millimole per liter). The numbers shown in the equation above are used to convert from mg/dL to mmol/L. For mmol/L, the equation would be:

2 x (Na+) + (Glucose) + (BUN) + (Ethanol)

Serum Osmotic Gap

Serum osmolality (measured) - serum osmolality (calculated)

Urine osmolality is frequently ordered along with serum osmolality. It is used to help evaluate the body's water balance and to investigate increased and decreased urine output. Increased urine output may be due to increased fluid intake, lack of appropriate amounts of ADH, or diabetes, with increased glucose levels leading to increased urine output. Decreased urine output may be due to a variety of causes, including decreased blood flow to the kidneys, an appropriate response to dehydration, or damage to tubular cells in the kidneys. Urine sodium and creatinine are often ordered along with urine osmolality. Sometimes a urine osmotic gap is calculated and used to help evaluate the kidney's ability to excrete acid and reabsorb bicarbonate, to detect the presence of osmotically active molecules, and to compare with the serum osmotic gap.

Stool osmolality may sometimes be ordered to help evaluate chronic diarrhea that does not appear to be due to a bacterial or parasitic infection or to another identifiable cause such as intestinal inflammation or damage. People with watery chronic diarrhea may have an osmotically active substance, such as a commercial laxative, that is inhibiting the reabsorption of water by the intestines. Sometimes a stool osmotic gap is calculated.

When is it ordered?

A serum osmolality test and osmotic gap may be ordered when a person has symptoms that a health practitioner suspects may be due to hyponatremia such as:

  • Excessive thirst
  • Confusion
  • Nausea
  • Headache
  • Lethargy
  • In severe cases, seizures or coma

These tests may also be ordered when it is suspected that someone has ingested a toxin such as methanol or ethylene glycol.

A urine osmolality test may be ordered along with blood testing when a health practitioner wants to compare urine results with the serum osmolality and/or when the person being tested is producing increased or decreased quantities of urine. Both may be ordered when is is suspected that the person may have diabetes.

Osmolality testing may be ordered for asymptomatic people with unexplained hyponatremia when a low sodium is discovered during testing for other reasons. Serum and urine osmolality testing may be ordered frequently to monitor the effectiveness of treatment for these conditions and at regular intervals to monitor those taking the drug mannitol.

Stool osmolality may be ordered when a health practitioner suspects that a person's chronic diarrhea may be due to an osmotically active substance.

What does the test result mean?

Osmolality is dynamic and will fluctuate as the body responds to and corrects temporary water imbalances. Serum and urine osmolality tests must be evaluated in the context of the person's clinical presentation and along with the findings of other tests, such as sodium, glucose, and BUN. Osmolality results are not diagnostic; they suggest that a person has an imbalance, but they do not pinpoint the cause.

In general, if someone has an increased serum osmolality, it is due to either decreased water in the blood or increased solutes. If the person has decreased serum osmolality, it is due to increased fluid levels.

When someone has an increased osmotic gap and is suspected of ingesting a toxin such as methanol, then it is likely that the person has done so, with the size of the gap related to the amount of toxin. During monitoring, if osmolality, the osmotic gap, and findings such as a low sodium level return to normal, then treatment has been effective.

With mannitol therapy, the monitoring goal is the maintenance of a stable "therapeutic" osmotic gap and the relief of cerebral edema.

Serum osmolality may be increased with:

  • Dehydration
  • Diabetes mellitus
  • hyperglycemia
  • Hypernatremia
  • Alcohol poisoning due to toxic ingestion of ethanol, methanol, ethylene glycol, or isopropyl alcohol
  • Uremia (accumulation of toxins in the blood)
  • Stroke
  • Head trauma
  • Kidney damage
  • Mannitol therapy
  • Shock

Serum osmolality may be decreased with:

  • Excess hydration
  • hyponatremia
  • Inappropriate ADH secretion

When a person has increased urine output and low osmolality, then the person either is ridding their body of excess fluids or is unable to concentrate urine appropriately. Increased urine output and a high osmolality may be seen when there is a substance being flushed from the body, such as excess glucose with diabetes. If a person has decreased urine output and high osmolality, then the person may be dehydrated; if someone has low or normal osmolality, the person may have kidney damage.

Urine osmolality may be increased with:

  • Dehydration
  • Congestive heart failure
  • Hypernatremia
  • Inappropriate ADH secretion
  • Adrenal insufficiency/Addison's disease
  • Liver damage
  • Shock

Urine osmolality may be decreased with:

  • Diabetes insipidus
  • Excess fluid intake
  • Hypercalcemia
  • Hypokalemia
  • Kidney failure
  • Kidney tubular damage

If someone has an increased stool osmolality gap, then it is likely that an osmotically active substance is causing their chronic liquid diarrhea. This can be seen with malabsorption and laxative abuse.

Is there anything else I should know?

A calculation is sometimes ordered for "free water clearance" to help evaluate the ability of the kidney tubules to appropriately concentrate and dilute urine. When urine osmolality is about the same as plasma osmolality, then free water clearance is zero. When blood volume decreases and urine is concentrated, then free water clearance will be negative. When fluid levels are increased and urine is dilute, then free water clearance will be positive.

What is being tested?

Osmolality is a measure of the number of dissolved particles in a fluid. The osmolality test reflects the concentration of substances such as sodium, potassium, chloride, glucose, and urea in a sample of blood, urine, or sometimes stool. It is used to evaluate the balance between water and dissolved particles in the blood and urine, to detect the presence of substances that may affect this balance, and to assess the kidneys' ability to concentrate urine.

Water balance in the body is a dynamic process that is regulated by controlling the amount of water excreted in the urine and by increasing or decreasing water drinking by regulating "thirst." Osmotic sensors in the body perceive and react to increases and decreases in the amount of water and particles in the bloodstream. When blood osmolality increases, indicating either a decrease in the amount of water in the blood or an increase in the number of particles, the hypothalamus secretes antidiuretic hormone (ADH), which tells the kidneys to conserve water. This results in more concentrated urine with higher urine osmolality and more dilute blood with lower osmolality. As blood osmolality decreases, ADH secretion is suppressed, the kidneys excrete increased amounts of dilute urine, the amount of water in the body decreases, and blood osmolality returns to normal.

Blood osmolality is primarily a measure of sodium dissolved in the serum. Sodium is the major electrolyte in the blood, urine, and stool. It works with potassium, chloride, and CO2 (in the form of bicarbonate) to maintain electrical neutrality in the body and acid-base balance. Sodium comes into the body in the diet and is normally conserved or excreted in the urine by the kidneys to maintain its concentration in the blood within a healthy range.

Urine osmolality primarily measures the waste products urea and creatinine. Urea and creatinine are produced and removed by the body at a relatively constant rate.

Glucose and urea are not electrolytes but as particles (molecules), they do contribute to osmolality. Normally their contributions are small, but when someone has high blood glucose (hyperglycemia, as found in diabetes) or high blood urea (seen in diseases such as kidney failure), their influence can be significant.

Glucose is osmotically active. This means it can draw water out of the body's cells, increasing the amount of fluid in circulation, which in turn increases the amount of dilute urine produced. Mannitol, a drug used to treat cerebral edema, also has this property. toxins such as methanol, isopropyl alcohol, ethylene glycol, propylene glycol, and acetone, and drugs such as acetylsalicylic acid (aspirin) can also affect osmolality when ingested in sufficiently large amounts.

Osmolality can be measured or it can be estimated from the major solutes expected to be in the blood. The difference between measured and calculated (estimated) results is called the "osmotic gap" or "osmolal gap." An increase in the osmotic gap (greater than 10) indicates the presence of other substances such as toxins, aspirin, or mannitol. The osmotic gap is sometimes used for evaluation when a person is being treated with mannitol or when someone is suspected of having ingested a toxin such as methanol or too much of a drug such as aspirin. Other common causes of an elevated osmolal gap are alcoholic ketoacidosis, kidney failure, diabetic ketoacidosis, and shock.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm. A random urine sample is collected using a clean catch method (see description under "Urinalysis: How is the sample collected for testing?"). A fresh (refrigerated or frozen within about 30 minutes of collection), liquid stool that is not contaminated by urine is collected in a clean container. Bacteria in the stool can change the results of the test within a short period of time.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation may be needed or you may be instructed to fast (nothing to eat or drink except water) for 6 hours before the test. Follow any instructions you are given. Because some medications can interfere with this testing, inform your health care provider of all of the medications you are taking, especially if you are taking mannitol.

  1. What is the difference between urine osmolality and specific gravity?

    Specific gravity is a common part of a urinalysis. It evaluates the weight of solids in water. Osmolality and specific gravity usually change in parallel to each other. When large and heavy molecules (such as glucose and protein) are present in the urine, however, the results will diverge. Specific gravity will be increased more, due to the weight of the molecules, while urine osmolality will be increased less, reflecting the number of molecules. Generally, osmolality is considered a more exact measurement of urine concentration than specific gravity.

  2. What is the difference between osmolality and osmolarity?

    Osmolality measures the concentration of solutes in a fluid by looking at the number of particles per weight (kilogram) of fluid. Osmolarity evaluates the number of particles per volume (liter) of fluid. With dilute fluids, they are essentially the same, but differences will be more noticeable at higher concentrations and care must be taken when comparing them as the units for osmolality and osmolarity are not the same.