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Refractometry (Blood Refractive Index)

Published: August 9, 2026Updated: August 16, 2026

This test measures the refractive index of your blood, which helps assess hydration status and certain blood conditions.

Quick summary

["A simple test to check how well your body is balancing fluids and electrolytes."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
18519-9
Short code
RefB
Synonyms
RefB
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
ekg
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

What Does This Test Measure?

Refractometry measures how light bends as it passes through your blood. This property is influenced by the concentration of solutes (like water and salts) in your blood. It’s often used alongside other tests to help diagnose conditions related to dehydration or overhydration.

Why Might Your Doctor Order This Test?

Your doctor may request this test if you have symptoms like extreme thirst, unusual fatigue, swelling, or changes in urine output. It can also be part of a broader evaluation for conditions affecting fluid balance, such as kidney issues or certain metabolic disorders.

How Is This Test Performed?

A small blood sample is taken from a vein, usually in your arm. The sample is placed in a refractometer—a device that measures how light bends through the blood. The result is a numerical value representing the refractive index, which is then interpreted by your healthcare provider.

What Are the Possible Results?

The refractive index reflects your blood’s overall solute concentration. While exact reference ranges can vary by lab, lower values may suggest dehydration, while higher values could indicate overhydration or other conditions affecting blood composition. Your doctor will compare the result to your clinical picture and other tests.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Frequently asked questions

What does a high or low refractive index mean?
A high refractive index might suggest overhydration or conditions that increase blood solute concentration, such as certain kidney disorders. A low index could indicate dehydration or loss of fluids. Your doctor will discuss the result in the context of your symptoms and other test results.
Can this test detect specific diseases?
No, this test alone does not diagnose specific diseases. It provides a general measure of blood solute concentration and is often used alongside other tests to support a diagnosis. For example, it may be part of an evaluation for kidney function or electrolyte imbalances.
How soon will I get my results?
Results from refractometry are typically available within a few hours to a day, depending on your lab’s processing time. Your doctor will review the results and share them with you during your follow-up visit.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy