LabCorp Test

Magnesium Blood Test, RBC

Quick Overview

The "gold standard" blood test for magnesium deficiency.

Test #080283


Availability: In stock

Also Known As Mg, RBC.
Preparation No special preparation required.
Test Results 2-3 days. May take longer based on weather, holiday or lab delays.


No fasting or other special preparation is required prior to giving blood for the red blood cell (RBC) blood test.  Individuals are able to take their medications and dietary supplements as they normally would the morning of the test.  Patients should let their doctor or the lab know before testing if they are taking a magnesium supplement, as this can affect test results.

Why It Is Used

The RBC blood test, also know as the magnesium blood test, is used to measure the level of magnesium found in the red blood cells floating in the blood serum.  The test is usually only ordered when there is reason to believe that an individual might have a magnesium deficiency.  Symptoms of low levels of magnesium in the red blood cells include cardiac arrhythmias, seizures, and muscle cramping and weakness. 

How It Works

The RBC blood test is more sensitive to magnesium deficiencies than a standard magnesium blood test.  When the body becomes magnesium deficient, it pulls the magnesium that is stored in red blood cells out of those cells and into the serum of the blood.  A standard blood test, therefore, may show magnesium levels as normal even though the red blood cell’s stores of magnesium are depleted.  Magnesium is crucially important for bone and muscle health.  Early detection and treatment of a magnesium deficiency helps to prevent the development of more chronic problems.


Lower than normal levels of magnesium in the blood are most commonly an indication of a dietary absorption problem that can result from conditions such as malnourishment, alcoholism, anorexia nervosa or bulimia nervosa, diabetes, gastrointestinal disorders, and prolonged use of diuretics.  Higher than normal levels of magnesium in the blood is usually due to a problem with the body not being able to properly dispose of excess magnesium.  This may be due to kidney failure, which results in a deceased ability to filter out substances from the blood.  It may also be due to a thyroid disorder, where the rate at which foods are metabolized is altered.  Sometimes an individual's magnesium levels are high because they take laxatives, antacids, or other medications that contain magnesium. 

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