Understanding Urine Osmolality: Normal Values and Their Significance

Urine osmolality measures how concentrated your urine is. A normal random urine sample can range widely, from 50 to 1200 mOsm/kg, because healthy kidneys make dilute urine when you drink a lot and concentrated urine when you are short of water. Low urine osmolality most often means you have been drinking plenty of fluid; less often it points to diabetes insipidus or kidneys that cannot concentrate urine. High urine osmolality most often reflects dehydration, and less often glucose in the urine or excess antidiuretic hormone (SIADH).

Blood (serum) osmolality is a different test with a much narrower normal range, 275–295 mOsm/kg. Low serum osmolality usually goes with low blood sodium; high serum osmolality usually reflects dehydration, high blood sugar or high sodium. Both are explained below.

What osmolality means

Osmolality is the number of dissolved particles – mainly sodium, glucose and urea – in a kilogram of water. It is reported in milliosmoles per kilogram (mOsm/kg). Laboratories using SI units write mmol/kg; the numbers are the same.

Your kidneys adjust urine concentration under the control of antidiuretic hormone (ADH, also called vasopressin). When ADH is high, the kidneys hold on to water and the urine becomes concentrated; when ADH is low, they release water and the urine becomes dilute. That is why a single urine value only makes sense next to your fluid intake, your urine volume and your blood results.

Normal values

Ranges vary slightly between laboratories, so compare your result with the range printed on your own report.

Low urine osmolality: what it means

The everyday reason and the medical ones:

• Drinking a lot of fluid – the kidneys correctly get rid of the extra water, so the urine is dilute. In a healthy person this is the everyday, harmless explanation.
• Low blood sodium (hyponatraemia) from drinking too much water – when blood is too dilute, healthy kidneys respond with very dilute urine, typically below 100 mOsm/kg. In many other kinds of low sodium, such as SIADH, fluid loss or heart failure, the urine stays more concentrated than expected, and that difference is what doctors use the test to find.
• Kidney failure or damage to the kidney tubules – damaged kidneys lose the ability to concentrate urine.
• Severe kidney infection (pyelonephritis).

Rarer:

• Diabetes insipidus – the body makes too little ADH, or the kidneys do not respond to it, so large volumes of dilute urine are passed even when the body needs water.

High urine osmolality: what it means

• Dehydration – the kidneys conserve water, so a small volume of concentrated urine is produced. This is the everyday explanation.
• Glucose in the urine – in uncontrolled diabetes, sugar is flushed out and raises urine concentration while urine volume stays high.
• Heart failure or shock – reduced blood flow makes the kidneys hold on to water.
• High blood sodium.
• SIADH – the body releases ADH when it should not, so urine stays concentrated even though the blood is dilute.
• Less common: adrenal insufficiency (including Addison disease) and narrowing of a kidney artery (renal artery stenosis).

Recent anaesthesia, IV contrast dye for a CT or MRI scan, some medicines (including dextran and sucrose) and herbal remedies can also affect the result, so tell your provider about them.

Serum osmolality: low and high

Low serum osmolality (below 275 mOsm/kg) means the blood is too dilute. The most common reasons are:

• Low blood sodium (hyponatraemia)
• Drinking too much water or fluid
• Too much ADH (SIADH), which can be caused by some lung conditions and medicines
• An underactive thyroid or adrenal gland

High serum osmolality (above 295 mOsm/kg) means the blood is too concentrated. Common reasons:

• Dehydration (water loss)
• High blood sugar
• High blood sodium
• A build-up of urea from kidney failure (uraemia)

Less common: diabetes insipidus, a stroke or head injury that reduces ADH, and poisoning with alcohols such as methanol or ethylene glycol.

Calculated osmolality and the osmolal gap

Many reports show a calculated osmolality worked out from your blood results rather than measured directly:

• In conventional units: 2 × sodium + glucose/18 + BUN/2.8 (glucose and BUN in mg/dL; some laboratories also add a term for alcohol)
• In SI units: 2 × sodium + glucose + urea (all in mmol/L)

Because sodium counts twice in this formula, a low calculated osmolality usually reflects a low sodium, and a high calculated osmolality usually reflects dehydration with high sodium, high glucose or high urea.

The osmolal gap is the measured value minus the calculated one. A gap above about 10–15 (the cut-off differs between laboratories) is considered raised and suggests extra particles that the formula does not count, such as methanol, ethylene glycol or other alcohols, or conditions such as alcoholic or diabetic ketoacidosis, kidney failure or shock.

Symptoms

An abnormal osmolality value causes no symptoms by itself. Symptoms come from the underlying problem: excessive thirst and passing very large amounts of dilute urine (often waking at night to urinate) with diabetes insipidus, and, with low blood sodium, sluggishness, confusion, nausea and loss of balance, followed by muscle twitching or seizures if it worsens. Symptoms of low sodium generally appear when effective plasma osmolality falls below about 240, and they are more severe when sodium falls quickly.

What usually happens next

Urine osmolality is almost never interpreted alone. A clinician typically looks at:

1. Serum osmolality, sodium, glucose and BUN (urea) taken at the same time.
2. Urine sodium and urine volume – for example, high urine osmolality with low urine volume suggests dehydration, while low or normal urine osmolality with low urine volume can suggest kidney damage.
3. Kidney function (creatinine), thyroid and adrenal tests when SIADH is suspected, because these must be normal before SIADH is diagnosed.
4. A water-deprivation test if diabetes insipidus is suspected: urine output, blood salts and weight are measured for about 12 hours without drinking, then the response to vasopressin is checked.

Frequently asked questions

What is a normal urine osmolality? On a random sample, 50 to 1200 mOsm/kg. After 12–14 hours without fluids, healthy kidneys should concentrate urine to above 850 mOsm/kg.

What does low urine osmolality mean? Your urine is dilute. Most often this simply follows drinking a lot of fluid. It can also be seen with low blood sodium, diabetes insipidus, kidney failure or a severe kidney infection.

What does high urine osmolality mean? Your urine is concentrated. The usual reason is dehydration; others include glucose in the urine, heart failure, high sodium and SIADH.

What does low serum osmolality mean in blood work? Your blood is more dilute than normal (below 275 mOsm/kg). This almost always goes with low sodium, from drinking too much water, SIADH, or an underactive thyroid or adrenal gland.

What does high serum osmolality mean? Your blood is more concentrated than normal (above 295 mOsm/kg), most often from dehydration, high blood sugar, high sodium or kidney failure.

What does "osmolality calc low" mean on my report? It is the calculated value, based mainly on sodium. A low result usually means your sodium is low; look at the sodium result on the same report.

Is urine osmolality the same as specific gravity? Both describe urine concentration. Osmolality counts the dissolved particles directly, while specific gravity compares the weight of urine with water; very dilute urine of below 100 mOsm/kg corresponds to a specific gravity below 1.003.

When to see a doctor

An unusual urine osmolality in a well person who has been drinking a lot or very little is often explained by that alone. It is worth discussing with your doctor soon if the result is repeated, if your blood sodium or serum osmolality is also abnormal, or if you have persistent heavy thirst and are passing large amounts of urine. Seek urgent care for confusion, fainting, a seizure, or if someone may have swallowed methanol, antifreeze (ethylene glycol) or another toxic substance.

Sources

1. Osmolality urine — MedlinePlus Medical Encyclopedia, U.S. National Library of Medicine
2. Osmolality blood test — MedlinePlus Medical Encyclopedia, U.S. National Library of Medicine
3. Osmolality and Osmolal Gap — Testing.com
4. Hyponatremia — Merck Manual Professional Version
5. Hyponatremia (Low Level of Sodium in the Blood) — Merck Manual Consumer Version
6. Central Diabetes Insipidus — Merck Manual Consumer Version
7. A retrospective analysis of glycol and toxic alcohol ingestion: utility of anion and osmolal gaps — BMC Clinical Pathology (PMC)

Decisions about your health should always be made in consultation with your healthcare provider.