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

A low serum (blood) osmolality means your blood is more dilute than usual: there is more water in it relative to dissolved particles, mainly sodium. It almost always goes together with a low blood sodium (hyponatremia), and common reasons include water pills (diuretics), the body holding on to too much water because of excess antidiuretic hormone (SIADH), heart failure, liver disease, kidney disease and drinking large amounts of fluid. A low urine osmolality is a different result: it means the urine is dilute, which is expected after drinking plenty, but can also point to diabetes insipidus or a kidney that cannot concentrate urine.

What osmolality measures

Osmolality is the number of dissolved particles in a fluid. In blood it mostly reflects sodium, with smaller contributions from glucose (sugar) and urea (a waste product made from protein). In urine it mostly reflects urea and creatinine. Your body keeps blood osmolality in a narrow range by adjusting thirst and antidiuretic hormone (ADH), which tells the kidneys how much water to keep.

Blood osmolality can be measured directly by the laboratory, or estimated ("calculated") from other results. Many reports show it as "calculated osmolality" or "osmolality calc". A common formula is:

• Calculated osmolality = 2 × sodium + glucose/18 + BUN/2.8 (sodium in mmol/L, glucose and blood urea nitrogen in mg/dL)
• In SI units: 2 × sodium + glucose + urea (all in mmol/L)

A low calculated osmolality usually just reflects a low sodium. A high calculated osmolality usually reflects a high sodium, a high glucose or a high urea.

Usual reference ranges

Ranges vary by laboratory and method, so always compare your result with the range printed on your own report. For osmolality, mOsm/kg and mmol/kg give the same number.

Urine osmolality has a very wide normal range because healthy kidneys make dilute urine when you drink a lot and concentrated urine when you are short of water. A single urine value means little on its own; it is read together with blood osmolality, blood sodium and how much urine you are passing.

Causes of low serum osmolality

Common

• Diuretics (water pills), especially thiazides – they make the kidneys lose sodium; older people are particularly prone to this.
• SIADH (syndrome of inappropriate antidiuretic hormone secretion) – too much ADH makes the kidneys keep water, diluting the blood. It can be linked to lung cancer and other conditions; several medicines can also make the body keep water - for example carbamazepine and opioids, and possibly some antidepressants (SSRIs) and anti-inflammatory painkillers.
• Heart failure, liver cirrhosis and kidney disease – the body retains water out of proportion to sodium.
• Diarrhea and vomiting – sodium is lost and often replaced with plain water.

Less common

• Drinking very large amounts of water or other fluids (primary polydipsia), including endurance athletes who replace sweat with plain water.
• Underactive thyroid (hypothyroidism) and adrenal gland insufficiency.
• Large burns or heavy sweating with sodium loss.

When a low sodium does not mean a low osmolality

• A very high blood sugar draws water out of cells and lowers sodium, but the osmolality is high, not low.
• Very high blood fats (triglycerides) or very high blood proteins (for example in multiple myeloma) can make sodium look falsely low on some analysers, while the measured osmolality stays normal. This is called pseudohyponatremia, a measurement artefact.

Causes of high serum osmolality

A high (raised) serum osmolality means the blood is more concentrated. Causes include dehydration (water loss), high sodium (hypernatremia), high blood sugar, high urea from kidney failure (uremia), diabetes insipidus, and a stroke or head injury that reduces ADH.

If the measured osmolality is clearly higher than the calculated one, the difference (the osmolal gap) suggests an extra substance in the blood that the formula does not count, such as alcohol (ethanol), methanol, ethylene glycol (antifreeze), isopropyl alcohol or the medicine mannitol. A raised gap is also seen in diabetic ketoacidosis, alcoholic ketoacidosis and shock, and can be seen in kidney failure, so it is not specific to poisoning.

Urine osmolality: low and high

• Low urine osmolality means dilute urine. It is normal after drinking a lot. It may also be linked to diabetes insipidus (the kidneys cannot respond to, or the body does not make enough, ADH), kidney failure, damage to the kidney tubules, or a severe kidney infection (pyelonephritis). In someone with low blood sodium, urine osmolality below 100 mOsm/kg suggests the kidneys are correctly getting rid of extra water, as happens with very high fluid intake.
• High urine osmolality means concentrated urine. It is expected when you are dehydrated, and it can also be linked to heart failure, SIADH, adrenal insufficiency, glucose in the urine, shock and narrowing of a kidney artery (renal artery stenosis).

Symptoms

The number itself does not cause symptoms; the speed and depth of the change do. A mildly low osmolality found on a routine test often causes no symptoms at all. When sodium and osmolality fall further, or fall quickly, water moves into brain cells and people may notice nausea, headache, tiredness, loss of appetite, muscle cramps or weakness, unsteadiness, falls and confusion. Symptoms generally appear when effective plasma osmolality falls below about 240 mOsm/L; seizures and coma can occur as sodium drops below about 115 mmol/L (mEq/L). Older people and people whose sodium falls quickly tend to have more symptoms.

What usually happens next

A clinician usually starts by repeating the blood test and looking at the result alongside:

• Blood sodium, potassium, glucose, urea (BUN) and creatinine – the ingredients of the calculated osmolality and a check on kidney function.
• Urine osmolality and urine sodium – to see whether the kidneys are holding on to water or losing salt.
• Thyroid and adrenal (cortisol) tests, especially if no other cause is obvious.
• Blood lipids and proteins, if pseudohyponatremia is suspected.
• A review of your medicines and how much you drink, and an assessment of whether you are dry, normal or overloaded with fluid.

Interpretation depends on the setting: dilute urine after drinking a lot means something different from dilute urine in someone with constant thirst and large urine volumes.

Frequently asked questions

What does low osmolality mean in blood work? It means your blood is more dilute than usual. It nearly always reflects a low sodium level, most often from diuretics, SIADH, heart, liver or kidney conditions, fluid losses replaced with water, or drinking a lot of fluid.

What is a normal urine osmolality? A random urine sample is usually somewhere between about 50 and 1,200 mOsm/kg, depending on how much you have been drinking. After 12–14 hours without fluids, healthy kidneys concentrate urine above 850 mOsm/kg.

What does a low calculated osmolality mean? Calculated osmolality is worked out from sodium, glucose and urea. When it is low, the usual reason is a low sodium, so the sodium result and its cause are what matter.

What does a high serum osmolality mean? Your blood is more concentrated than usual, most often from dehydration, high sodium or high blood sugar. Kidney failure, diabetes insipidus and certain toxic substances can also raise it.

Is low serum osmolality dangerous? A mild, stable value often causes no symptoms. A large or rapid fall can cause brain swelling, confusion, seizures and, rarely, coma, so the cause should always be found.

Is low serum osmolality the same as dehydration? No. Dehydration usually raises serum osmolality. A low value points to too much water relative to sodium, not too little.

When to see a doctor

If a routine test shows a mildly low or high osmolality and you feel well, it is worth discussing the result with your doctor soon so the sodium, kidney function and possible medicine effects can be checked. Bring a list of your medicines, including water pills and antidepressants. Do not stop any medicine on your own.

Seek urgent care if a low sodium or osmolality comes with confusion, severe headache, repeated vomiting, fainting, a seizure or unusual drowsiness, or if you suspect someone has swallowed antifreeze, methanol or another chemical.

Sources

1. Osmolality blood test — MedlinePlus, U.S. National Library of Medicine
2. Osmolality urine test — MedlinePlus, U.S. National Library of Medicine
3. Low blood sodium — MedlinePlus, U.S. National Library of Medicine
4. Hyponatremia — Merck Manual Professional Version
5. Hyponatremia — MSD Manual Professional Version
6. Osmolality and Osmolal Gap — Testing.com
7. An evaluation of the osmole gap as a screening test for toxic alcohol poisoning — BMC Emergency Medicine (PMC)

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