Refractive index of serum or plasma (measured by refractometry)

What Does This Test Measure?

Refractometry measures how much light bends (refracts) when passing through your blood or fluid sample. This property is influenced by the concentration of solutes, such as proteins, electrolytes, or glucose. While not a direct measurement of a single substance, it indirectly reflects overall solute density, which can be useful in diagnosing conditions like dehydration, hyperproteinemia, or certain metabolic disorders.

Why Might Your Doctor Order This Test?

Your doctor may request this test to evaluate hydration status, check for abnormal protein levels, or monitor conditions affecting blood composition. It’s sometimes used alongside other tests to confirm or refine diagnoses.

Common Uses and Clinical Contexts

This test is often used in clinical settings where quick, general assessments of blood or fluid density are needed. It may be part of a broader panel when evaluating symptoms like fatigue, swelling, or unexplained weight changes.

How Is This Test Performed?

A small blood sample is taken from a vein, usually in your arm. The sample is then placed in a refractometer—a device that measures how light bends through the fluid. The result is a numerical value representing the refractive index, which reflects the overall solute concentration.