RDW Blood Test

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What is an RDW (red blood cell distribution width) blood test?

An RDW (red blood cell distribution width) blood test measures how varied your red blood cells are in size and volume. Your red blood cells carry oxygen from your lungs to organs and tissues throughout your body. The oxygen fuels your body’s cells so that they can produce energy.

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When it comes to your red blood cells, size matters. Healthy red blood cells are about the same size, ranging from 6.2 to 8.2 micrometers. Having red blood cells that are vastly different sizes from each other (high variation) may be a sign of anemia. Anemia is a condition that involves not having enough healthy red blood cells to supply oxygen to your body’s organs.

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An RDW blood test is just one lab test your healthcare provider may use to diagnose anemia or other conditions.

What is the difference between an RDW blood test and a complete blood count (CBC)?

Your healthcare provider may order an RDW blood test as part of a more extensive blood workup called a complete blood count (CBC). A CBC is a blood test that provides information about multiple parts of your blood, including red blood cells, white blood cells and platelets.

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One portion of a CBC includes a panel of tests called RBC (red blood cell) indices. RBC indices provide information about your red blood cells in particular. An RDW blood test is part of the panel.

RBC indices include:

Red cell distribution width (RDW): Measures the size variation of your red blood cells.
Mean corpuscular volume (MCV): Measures the average size of your red blood cells.
Mean corpuscular hemoglobin (MCH): Measures the average amount of hemoglobin in your red blood cells. Hemoglobin is a protein that allows your red blood cells to transport oxygen throughout your body.
Mean corpuscular hemoglobin concentration (MCHC): Measures the average amount of hemoglobin that’s concentrated (close together) in your red blood cells.

Together, these tests may be used to diagnose various conditions associated with problems involving your red blood cells.

What does an RDW blood test measure?

The word “width” in red cell distribution width can be confusing. It’s easy to assume that the test measures the width of your red blood cells, but this isn’t the case. Instead, an RDW blood test measures the variation in cell size among a representative sample of your red blood cells.

Cell distribution width refers to the distance between cell sizes as they’re plotted on a histogram. A histogram is a special type of graph that shows a range of numbers or values, like the range of red blood cell sizes. Similar values (more or less the same cell size) will be clustered close together on the histogram, and the RDW will be low. Different values (varied cell sizes) will be spread out on the histogram, showing a wider distribution of values. In this case, the RDW will be high.

When would an RDW blood test be needed?

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An RDW blood test can help your healthcare provider get closer to a diagnosis if you have anemia-like symptoms or if you have a condition associated with anemia.