Preferred Sample Type


Suitable Specimen Types

  • Serum
  • EDTA Plasma
  • Li Hep Plasma
0.5 mL blood

Sample Processing in Laboratory


Sample Preparation

Centrifuge and separate. All serum and urine samples must be analysed within 24 hours of receipt into the laboratory.

Turnaround Time

1 Day

Sample Stability

4 ºC


General Information

The osmolality test is a snapshot of the number of solutes present in the blood (serum), urine, or stool. It is ordered to help evaluate the body's water balance, its ability to produce and concentrate urine, to help investigate low sodium levels (hyponatremia), to detect the presence of toxins such as methanol and ethylene glycol, and to monitor osmotically active drug therapies such as mannitol. It is also ordered to help monitor the effectiveness of treatment for any conditions found.

Serum osmolality is primarily used to investigate hyponatremia. Hyponatremia may be due to sodium loss through the urine or due to increased fluid in the bloodstream. Increased fluid may be due to drinking excessive amounts of water, water retention, decreased ability of the kidneys to produce urine, or the presence of osmotically active agents such as glucose, mannitol, and glycine (a chemical used in surgical irrigation fluids).

Mannitol, glycine, and the ingestion of toxins such as methanol and propylene glycol can be detected, evaluated, and monitored by ordering an osmolal gap. This calculation compares measured osmolality with measurements of the major solutes. The osmotic gap is the difference between them and represents the presence of an osmotically active substance in the blood. In order to calculate the osmotic gap, tests for blood sodium, blood urea and glucose must be performed.

Patient Preparation



The osmolar gap can be determined using:

Osmolar gap = Measured osmolality - calculated osmolality

Calculated osmolality is determined using:

Calculated osmolality = (2 x Na) + (2 x K) + Glucose + Urea (all in mmol/L)

Please contact the laboratory if urgent

Reference Range

275-295 mOsmol/kg

(Source : Pathology Harmony Recommendations)


  • EQA Scheme?: Yes
  • EQA Status:


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The Trust Laboratories at Heartlands Hospital, Good Hope Hospital and Solihull Hospital were awarded UKAS (United Kingdom Accreditation Service) accreditation to the internationally recognised ISO 15189 standard in May 2015. For a list of accredited tests and other information please visit the test database
Tests not appearing on this scope are either under consideration or in the process of accreditation and so currently remain outside of our scope of accreditation. However, these tests have been validated to the same high standard as accredited tests and are performed by the same trained and competent staff.

For further information contact Louise Fallon, Quality Manager, 0121 424 1235

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