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What is Serum Osmolality test? 

The serum osmolality test measures the concentration of dissolved chemicals and electrolytes (especially sodium) in your serum. Serum is the liquid part of the blood that is left after removing blood cells and clotting proteins. A perfect water-electrolyte balance is important to maintain homeostasis in the body and for body cells to function properly. 

However, blood osmolality changes according to the amount of water and electrolytes in your body including sodium, chloride, bicarbonate, proteins, and glucose. When the amount of water in the blood is less, the concentration of electrolytes (serum osmolality) increases, and vice versa. 

Your body controls serum osmolality with the help of a hormone called antidiuretic hormone (ADH). Every time you are dehydrated, it produces ADH to prevent the loss of water through urine.

On the other hand, if you consume a lot of water, the serum osmolality decreases and your body stops the release of ADH to promote the expulsion of excess water in the urine and restore the osmotic balance.

Some chemicals like ethanol and certain toxic substances such as methanol, isopropyl alcohol, ethylene glycol or propylene glycol can also be detected in the blood with this test. 

  1. Why is a Serum Osmolality test performed?
  2. How do you prepare for a Serum Osmolality test?
  3. How is a Serum Osmolality test performed?
  4. Serum Osmolality test results and normal range

Why is a Serum Osmolality test performed?

The test is performed to check if the balance of water and chemicals in your body is stable (in homoeostasis). Your doctor may order this test in one of the following conditions if he/she has a reason to believe that this delicate balance is lost:

  • Seizures and coma (due to severe imbalance between electrolytes and water in the body)
  • Problems with ADH
  • Dehydration
  • If your doctor thinks you may have diabetes insipidus (a condition in which large volumes of urine are produced) 
  • To determine if you have ingested any poison like isopropyl alcohol, methanol or ethylene glycol
  • Hyponatraemia, a condition in which the body retains fluids due to sodium deficiency

Hyponatraemia can show the following symptoms:

In some cases, the test may also help in diagnosing toxic alcohols in the blood like methanol, isopropyl alcohol and ethylene glycol.

How do you prepare for a Serum Osmolality test?

Notify your doctor if you are taking any drugs (both prescription and non-prescription), supplements and vitamins. This is because a lot of drugs may affect the results of this test. Your healthcare provider may request you to stop the use of some medicines. Do not stop their use on your own. 

Do not drink excessive amounts of water before the test and refrain from consuming alcohol just before the test. Avoid intense exercise and or stressful situations before going for this test. 

If you have recently had a blood transfusion, you may have to wait for a while before getting the test as you may get inaccurate results.

How is a Serum Osmolality test performed?

Serum osmolality test is performed on a blood sample. Your doctor or a lab technician will withdraw the required amount of blood from a vein in your arm and immediately send it to the laboratory for testing.

Some risks associated with blood tests are:

If you experience persistent discomfort, check in with your doctor at the earliest.

Serum Osmolality test results and normal range

Normal results:

The normal values for Serum osmolality are:

  • For adults: 275-295 mOsm/kg (milliosmoles/Kg)
  • For children: 275-290 mOsm/kg

Abnormal results:

Various conditions may cause abnormal serum osmolality values. 

Serum osmolality values above the normal range may be due to:

  • Hypernatraemia (high levels of sodium in the blood) due to dehydration
  • Kidney damage leading to urea build-up in the serum
  • Poisoning with chemicals such as
    • Ethanol (in alcoholic beverages)
    • Methanol (wood alcohol)
    • Isopropyl alcohol (rubbing alcohol)
    • Ethylene glycol (antifreeze)

Sometimes, the difference between the calculated value and the result will be greater than 10. This difference is called the osmolal gap and is indicative of poisoning with chemicals as above. 

  • Diabetes insipidus
  • Conditions where salt and sugar levels in the blood are high, such as poorly controlled diabetes
  • Hypernatraemia due to iatrogenic (caused by treatment or medical examination) or accidental intake of sodium chloride or sodium bicarbonate

Test results may be below the normal range for the following reasons:

  • Presence of excess water in the body
  • Hyponatraemia (low sodium in the blood) due to some medicines, such as diuretics and certain blood pressure medications
  • Syndrome of inappropriate antidiuretic hormone secretion (SIADH) that may occur with 
  • Hyponatraemia due to disorders such as
    • Psychogenic polydipsia (abnormal thirst)
    • Hypothyroidism (an underactive thyroid gland)
    • Adrenal insufficiency (insufficient production of hormones such as cortisol and sometimes aldosterone)
    • Cirrhosis
    • Congestive heart failure (compromised pumping capacity of heart muscles)
    • Nephrotic syndrome 

Abnormal values may also be due to:

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor.

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