Why is Serum Osmolality High? Causes, Symptoms, and What to Do

High serum osmolality means the fluid part of your blood is more concentrated than usual: there is too little water for the amount of dissolved particles, mainly sodium, glucose and urea. The most common reasons are losing more water than you drink (dehydration), high blood sugar and a high blood sodium level; less often it reflects diabetes insipidus, kidney failure or a swallowed substance such as alcohol or antifreeze. Low serum osmolality means the opposite, too much water for the particles, and is usually linked to low sodium. A normal blood (serum) osmolality is about 275–295 mOsm/kg, while a random urine sample can normally be anywhere from 50 to 1,200 mOsm/kg depending on how much you drink.

What the test measures

Osmolality is the concentration of all dissolved particles in a kilogram of water. The blood test helps check your body's water balance, low sodium, poisoning, and problems with thirst or urine production. Your brain watches this number closely: when it rises, you feel thirsty and release antidiuretic hormone (ADH, also called vasopressin), which makes the kidneys hold on to water; when it falls, thirst and ADH are switched off.

Usual adult reference values

The two units, mOsm/kg and mmol/kg, give the same number. Ranges vary slightly between laboratories, so compare your result with the range on your report.

"Osmolality, calculated" on a report is an estimate worked out from your sodium, glucose and urea (BUN) results rather than measured directly. If the measured value is higher than the calculated one by 10 mOsm/kg or more, something not included in the formula is adding particles, most commonly an alcohol, mannitol or glycine.

Causes of high serum osmolality

Common

• Dehydration (water loss greater than intake). Less water in the blood concentrates everything dissolved in it. In older adults, limited access to water, a weaker thirst sense and kidneys that concentrate urine less well all contribute.
• High blood sugar (hyperglycemia). Glucose itself adds particles, and in diabetes high glucose spilling into the urine pulls water out with it (osmotic diuresis).
• High blood sodium (hypernatremia, sodium above 145 mmol/L). Sodium is one of the main particles in blood, so a high sodium raises osmolality. Hypernatremia usually involves an impaired thirst mechanism or limited access to water.
• Alcohol (ethanol). Alcohols are the most common of the unmeasured particles that raise measured osmolality above the calculated value.

Less common

• Uremia. A build-up of nitrogen waste products when the kidneys fail.
• Diabetes insipidus. Too little ADH, or kidneys that do not respond to it, so large amounts of dilute urine are lost.
• Stroke or head injury that reduces ADH release.
• Poisoning with methanol or ethylene glycol (antifreeze); the osmolal gap is used to look for these.

Causes of low serum osmolality

Low osmolality means water excess relative to particles. Listed causes include drinking too much water or fluid, low blood sodium (hyponatremia, sodium below 136 mmol/L), the syndrome of inappropriate ADH secretion (SIADH, which can be linked to lung cancer, other conditions or medicines), adrenal insufficiency and an underactive thyroid. Thiazide diuretics reduce the kidneys' ability to dilute urine and can lower sodium.

A low sodium is not always truly "dilutional". Very high glucose, or very high blood fats or proteins, can make sodium read low; in the second case the measured osmolality stays normal. This is one reason doctors measure osmolality alongside sodium.

Urine osmolality: what low and high mean

Urine osmolality is read together with how much urine you pass and your blood results.

• Low urine osmolality with plenty of urine fits drinking a lot or kidneys that cannot concentrate urine. MedlinePlus lists diabetes insipidus, excessive fluid intake, kidney failure, low blood sodium, renal tubular necrosis and severe kidney infection. With low blood osmolality, urine below 100 mOsm/kg is the expected, appropriate response.
• High urine osmolality with little urine usually suggests dehydration. Other listed causes are glucose in the urine, heart failure, high sodium, adrenal insufficiency, renal artery stenosis, shock and SIADH.

Symptoms

A number on a report does not cause symptoms; the underlying water or sodium problem does. With high osmolality and high sodium, thirst is the main symptom; as brain cells lose water, confusion, muscle twitching, seizures or coma can follow. With low sodium, symptoms include headache, nausea, lethargy and confusion, with seizures in severe cases. Symptoms tend to be worse when the change happens quickly.

What usually happens next

Doctors look at serum osmolality together with:

• Sodium, glucose and BUN (urea), needed to calculate osmolality and the osmolal gap;
• Urine osmolality, urine sodium and creatinine, to see how the kidneys are handling water;
• Thyroid and adrenal tests when osmolality and sodium are low, since an underactive thyroid and adrenal insufficiency are listed causes;
• Urine volume, and sometimes a supervised water-deprivation test, when diabetes insipidus is suspected.

Frequently asked questions

What does high serum osmolality mean? Your blood has too little water compared with dissolved particles. Dehydration, high blood sugar and high sodium are the usual reasons; alcohol, kidney failure and diabetes insipidus are others.

What does low osmolality mean in blood work? Your blood is more diluted than usual. It mostly goes with low sodium, which may come from drinking a lot of fluid, SIADH, certain diuretics, an underactive thyroid or adrenal problems.

What does "osmolality calculated" low or high mean? The calculated value is estimated from sodium, glucose and urea, so it is low or high for the same reasons those results are. Its main use is comparison with the measured value: a gap of 10 mOsm/kg or more points to an extra substance such as an alcohol.

What is a normal urine osmolality? A random urine sample can range from 50 to 1,200 mOsm/kg water depending on how much you drink. After 12–14 hours without fluids, healthy kidneys concentrate urine above 850 mOsm/kg water.

What does low urine osmolality mean? Your urine is dilute. Often this is simply from drinking a lot, but it can also reflect diabetes insipidus, kidney damage or kidney infection, so it is interpreted with your blood results.

Do you need to fast for a serum osmolality test? Usually no special preparation is needed, but your clinician may ask you not to eat beforehand or to limit fluids, and may ask you to pause medicines such as diuretics. Follow your own instructions and do not stop a medicine on your own.

When to see a doctor

It is worth discussing an abnormal osmolality result with your doctor soon, especially if it is repeated, if you have diabetes, kidney disease or take diuretics, or if you notice constant thirst or passing very large amounts of urine. Seek urgent care for confusion, fainting, a seizure, or if someone may have swallowed methanol, antifreeze or another toxic substance.

Sources

1. Osmolality blood test — MedlinePlus (U.S. National Library of Medicine)
2. Osmolality urine test — MedlinePlus (U.S. National Library of Medicine)
3. Osmolality and Osmolal Gap — Testing.com
4. Water and Sodium Balance — Merck Manual Professional Version
5. Hypernatremia — Merck Manual Professional Version
6. Hyponatremia — Merck Manual Professional Version

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