Osmolality: Definition, Calculations & Formula

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  • 0:00 What Is Osmolality?
  • 0:48 Calculating Osmolality
  • 2:51 Interpretation
  • 4:19 Lesson Summary
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Lesson Transcript
Instructor: Lacey Russell

Lacey has a Master's of Science in Nursing with a specialization in Acute Care Nurse Practitioner

In this lesson, we will discuss osmolality. First, we will define it and then explain how it is calculated and what is considered the normal range. Lastly, we will discuss what the findings of the test indicate.

What Is Osmolality?

Serum osmolality is a lab test that tells a health care provider how much water there is in the blood compared to how many solutes are in the blood. Solutes are particles or chemicals in the bloodstream. What's great about this test is that it can analyze the fluid part of the blood and measure the exact concentration of chemical particles. This test is most commonly used to assess for the presence of hyponatremia, or low sodium levels in the blood.

Urine osmolality is another lab test that does the same thing except it measures the amount of solutes dissolved in the water of the urine rather than in the blood. This test is commonly ordered to help assess urinary concentration and dilution. It can also help assess the hydration status of a patient.

Calculating Osmolality

Serum osmolality is calculated using the results from several other lab tests include sodium, blood urea nitrogen (BUN), and glucose. The formula is:

Serum Osmolality = 2 x (Na+) + (Glucose) + (BUN)

This formula assumes that the values being input are in millimoles per liter (mmol/L). If they're reported in milligrams per deciliter (mg/dl), the following formula would be used:

Serum Osmolality = 2 x (Na+) + (Glucose/18) + (BUN/2.8)

Let's try it out with some real values with this formula, since most lab results are given in mg/dl. The patient has a sodium of 130, glucose of 86, and BUN of 1.

Serum Osmolality = 2 x (130) + (86/18) + (1/2.8) = 260 + 4.78 + 0.36 = 265.14

The normal range for serum osmolality is 285 to 295 mOsm/kg. As you can see, the patient's osmolality was low, probably because his sodium was low.

Urine osmolality is calculated with the following formula:

Urine Osmolality = 2 x (urine Na) + Urine K + (urinary urea nitrogen/2.8) + (urine glucose/18)

Let's do a real life application of this one as well. The patient has a urine sodium level of 200, urine potassium of 100, urine nitrogen of 20, and a urine glucose of 1.

Urine Osmolality = 2 x (200) + 100 + (20/2.8) + (1/18) = 400 + 100 + 7.14 + 0.06 = 507.20

The normal urine osmolality of a 24-hour urine test should be 500 to 800 mOsm/kg.


Usually, if a patient has an increased serum osmolality, it's caused by either a decreased amount of water in the blood or an increased amount of solutes. If the patient has a decreased serum osmolality, it's associated with increased fluid levels.

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