Newcastle Laboratories

Osmolality, serum

Clinical Background:

Osmolality is a measure of the dissolved solutes in blood. The main constituents are sodium, potassium, chloride, bicarbonate, urea and glucose. Changes in water content or the major contributors w...

Osmolality is a measure of the dissolved solutes in blood. The main constituents are sodium, potassium, chloride, bicarbonate, urea and glucose. Changes in water content or the major contributors will alter osmolality in serum. Normally this would be controlled by alterations in ADH secretion and thirst to maintain serum osmolality in a tight range and results in a varying urine osmolality. When this mechanism is overridden, then changes to the serum osmolality can be seen. Other osmotic agents such as ethanol, ethylene glycol and methanol can also increase serum osmolality and would produce an osmolal gap.

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Test Details

  • Discipline:

    Biochemistry

    Biochemistry

  • Specimen Container Adult:

    Serum-SST

    Serum-SST

  • Specimen Container Paediatric:

    Serum-Plain tube

    Serum-Plain tube

  • Minimum Volume Adult:

    1 mL blood

    1 mL blood

  • Minimum Volume Paediatric:

    0.5 mL blood

    0.5 mL blood

  • Sample Stability:

    Unseparated sample: Unknown

    Separated samples:

    12 hrs at 20-25C

    3 days at 4-8C

    3 months at -20C

    Unseparated sample: Unknown

    Separated samples:

    12 hrs at 20-25C

    3 days at 4-8C

    3 months at -20C

  • Interpretation:

    Usually interpretation of serum osmolality requires knowledge of the fluid status of the patient and urine osmolality/sodium. Some of the causes of changes in osmolality include: diabetes insipidus...

    Usually interpretation of serum osmolality requires knowledge of the fluid status of the patient and urine osmolality/sodium. Some of the causes of changes in osmolality include: diabetes insipidus, SIADH, primary polydipsia, osmotic diuresis, acute renal injury, hypoadrenalism.
    In poisoning with ethanol, ethylene glycol or methanol an osmolal gap often exists. This can be established by calculating the osmolality = 2*(Na+K) + urea + glucose and subtracting from the measured value. This should be done before considering measurement of the agents.
    For further information on interpreting osmolality please contact the duty biochemist.

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  • Reference Ranges:

    275-295 mmol/Kg 

    275-295 mmol/Kg 

  • Routine Contact Name:

    Duty biochemist

  • Routine Telephone:

    Freeman: 0191 244 8889
    RVI: 0191 282 9719

    Freeman: 0191 244 8889
    RVI: 0191 282 9719

  • Routine Email:

Availability:

All times
All sites

Turn Around:

Within 1 day

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