What is the Osmolality (Serum) test?

The Osmolality (Serum) test measures the concentration of dissolved particles in the blood, including sodium, glucose, and urea. It reflects how concentrated or diluted your blood is and plays a key role in fluid balance regulation.

Serum osmolality is tightly controlled by the kidneys and hormones such as antidiuretic hormone (ADH). Changes in this value can indicate dehydration, overhydration, electrolyte imbalance, or underlying kidney or endocrine disorders.

This test is commonly used alongside electrolyte panels and urine osmolality to help determine the cause of fluid balance disturbances.

Why do I need a test for Osmolality (Serum)?

Serum osmolality is essential when investigating symptoms related to fluid imbalance or abnormal electrolyte levels. It provides valuable insight into how well your body is regulating water and solutes.

This test is often requested when there are concerns about dehydration, excessive fluid intake, kidney disease, diabetes, or hormonal imbalances affecting water regulation. It can also support diagnosis in cases of unexplained neurological symptoms.

You may benefit from this test if you experience:

• Persistent thirst or excessive fluid intake

• Dehydration or fluid retention

• Confusion or altered mental state

• Headaches or dizziness

• Nausea or vomiting

• Unexplained fatigue

• Abnormal sodium or glucose levels

• Suspected kidney or endocrine disorders

Benefits of the Osmolality (Serum) test

1. Assesses hydration status
   Helps determine whether the body is dehydrated or overhydrated.

2. Evaluates electrolyte balance
   Supports interpretation of sodium, glucose, and urea levels.

3. Supports kidney function assessment
   Provides insight into the kidneys’ ability to regulate fluids.

4. Aids investigation of hormonal disorders
   Useful in assessing conditions involving antidiuretic hormone imbalance.

5. Supports diagnosis of fluid imbalance causes
   Helps distinguish between different causes of abnormal sodium levels.

6. Assists in neurological symptom evaluation
   Abnormal osmolality can affect brain function and mental status.

7. Useful in diabetes and metabolic assessment
   Elevated glucose can significantly influence serum osmolality.

8. Guides clinical decision-making
   Helps clinicians determine appropriate fluid or electrolyte management.

Step-by-step – how we offer Osmolality (Serum) testing

Clinic-based testing (UK-wide)

• Choose your nearest UK clinic

• Attend your scheduled appointment

• A qualified clinician or phlebotomist collects your blood sample

• Sample is sent to the laboratory for analysis

• Results are issued once testing is complete

At-home or hotel testing (+£60)

• A qualified phlebotomist visits your home or hotel

• Blood sample is collected in a comfortable setting

• No travel or clinic visit required

• Ideal for busy individuals or limited mobility

Both services are available UK-wide.

Your Osmolality (Serum) results explained

Low serum osmolality

Low serum osmolality indicates diluted blood and is often associated with overhydration or conditions affecting water regulation. It may occur when excess fluid intake dilutes electrolytes or when hormonal regulation is impaired.

Common causes include hyponatraemia, syndrome of inappropriate antidiuretic hormone secretion (SIADH), and certain medications.

UK reference range:

• Low: <275 mOsm/kg

Normal serum osmolality

Normal values indicate balanced fluid and solute levels in the bloodstream. This suggests appropriate kidney function and effective hormonal control of water balance.

Symptoms may still occur if fluid balance is fluctuating or if other related markers are abnormal.

UK reference range:

• Normal: 275–295 mOsm/kg

High serum osmolality

High serum osmolality reflects concentrated blood and is commonly linked to dehydration, high sodium levels, or elevated glucose. This can place stress on the kidneys and affect neurological function.

Causes may include dehydration, uncontrolled diabetes, excessive sodium intake, or fluid loss through vomiting or diarrhoea.

UK reference range:

• High: >295 mOsm/kg