What Is a RDW Blood Test?
An RDW blood test – also known as a red blood cell distribution width blood test – is a measure of the difference in your red blood cells’ size and volume. RDW is one part of a typical full blood workup called a complete blood count, or CBC. It’s one of the values doctors use to better understand the health of your circulatory system and, in some cases, diagnose disease.
What does RDW mean in a blood test?
Even though the test includes the word “width,” it doesn’t measure the width of your red blood cells. “Distribution width” is a range or spread of values. It shows how much variation there is between the highest value and the lowest value. In the case of RDW, these values are the volume and size of your red blood cells.
Your body makes red blood cells (erythrocytes) in bone marrow, the spongy tissue in the center of your bones. A red blood cell’s main job is to carry oxygen from your lungs to the tissues in your body. Your cells use the oxygen to grow, divide, and make energy for your body. Red blood cells also carry carbon dioxide waste away from the tissues back to the lungs. They use a protein called hemoglobin to do this.
An average red blood cell is about 6.2 to 8.2 micrometers. Typically, red blood cells are all similar in size. When red blood cells are vastly different sizes in your body, this can affect how well they’re able to do their job.
CBC vs. RDW blood test
CBC, or complete blood count, is a common blood test doctors use to look at your overall health and diagnose or monitor medical conditions. A CBC looks at all the parts of your blood, including red blood cells, white blood cells, and platelets.
A CBC includes a panel of tests called red blood cell (RBC) indices. These tests look specifically at your red blood cells. RDW is included in this panel. The other tests include:
- Mean corpuscular volume (MCV), which measures the average size of your red blood cells
- Mean corpuscular hemoglobin (MCH), which measures the average amount of hemoglobin in your red blood cells
- Mean corpuscular hemoglobin concentration (MCHC), which measures the average amount of hemoglobin in your red blood cells
Your doctor looks at the results of your CBC to see if you fall outside typical ranges for these tests. If you have a result outside the typical range, this can give your doctor an idea of what needs to happen next. You may need a specialist or a different treatment, or the results may help them make a diagnosis. Sometimes your doctor just keeps an eye on these values over time.
Who Gets an RDW Blood Test?
RDW tests are always included in CBC tests. The most common reason your doctor would look more closely at RDW results would be if they suspect you could have anemia. Anemia is a condition where your body doesn’t make enough red blood cells.
Symptoms of anemia include:
- Fatigue
- Shortness of breath
- Feeling cold
- Dizziness
- Weakness
- Pale or dry skin
- Headache
- Problems with your heart rhythm or rate (arrhythmia)
Sometimes, RDW results can also help your doctor figure out what’s causing your anemia.
You may also need RDW results if you have:
- A family history of thalassemia, sickle cell anemia, or another inherited blood disorder that could be causing your anemia
- A chronic illness such as Crohn's disease, diabetes, or HIV
- A diet low in iron and other minerals
- A long-term infection
- A lot of blood loss from an injury or a surgery
RDW helps your doctor monitor these conditions and assess your nutrition.
Is it safe?
Getting a blood test is a safe procedure. There is a small chance you could have some issues, including:
- Bleeding
- Infection
- Bruising
- Feeling lightheaded
- A sting of pain when the needle goes in
- Soreness at the stick site
What to Expect From a Red Blood Cell Distribution Width Blood Test
Typically, there isn’t any special preparation you need to do before a blood test. For a CBC only, you can eat and drink normally beforehand. Ask your doctor if there are other readings they'll do that require you to fast before your blood test.
How RDW blood tests are done
To take your blood, a technician will put a needle into a vein in your arm. Usually, they’ll choose a vein in the crook of your elbow. They fill a vial (sometimes more) with your blood. Then they remove the needle and place a bandage on the site to absorb any bleeding afterward.
Next, they'll send the blood sample off to a lab.
After the test
Once you’re done with your blood draw, you can go home. A blood test doesn’t require any recovery, unless you feel lightheaded and need a few minutes to feel normal again.
RDW Blood Test Results
Your RDW results will come in a special graph called a histogram. When your red blood cells are close to the same size, they appear close together on the histogram. This gives you a low RDW result. If your red blood cells are very different in size, they’ll be far apart on the histogram, and your RDW test result will be high.
There are two types of RDW measurements:
- Standard RDW is the most common type of RDW measurement labs use. It tells your doctor the overall variation in RBC size.
- Coefficient of variation (CV) RDW is more detailed and gives information about the average size of your RBCs and how widely the measurements vary.
Your doctor will need more information than just RDW to diagnose you with a condition. Often, they read RDW results alongside mean corpuscular volume (MCV) test results. The combination of these numbers can give a more accurate picture of what’s going on.
High RDW
If you get a “high” result on an RDW test, it means your red blood cells measure at vastly different sizes than the standard. This can be a sign of anemia or other conditions related to anemia.
“When the RDW is increased and there is a microcytic anemia [anemia caused by small, abnormally sized red blood cells], this is often suggestive of iron-deficiency anemia,” says Brady Stein, MD, a hematologist at Northwestern Medicine.
Iron-deficiency anemia happens when a lack of iron in your body keeps you from making enough blood cells.
A high RDW may also help a doctor figure out if you lack nutrients such as a folate or vitamin B12.
Studies show that there is a link between high RDW levels and worse outcomes in people with cardiovascular disease, brain vascular disease, strokes, sepsis, chronic obstructive pulmonary disease, and hepatitis B.
Low RDW
A low RDW means your red blood cells are similar in size and close together. This is not a sign of anemia and isn't something to worry about.
Average RDW
An average or normal RDW means your red blood cells are within the typical range for your sex and age. In other words, they’re all about the same size. Normal RDW ranges from about 12% to 15%, depending on the lab that does the test. But even with a normal result, your red blood cells may still be larger or smaller than usual, which can help diagnose certain problems.
“When a patient has microcytosis – which means small red cells – but a normal RDW, this can be one indication for a hereditary condition such as thalassemia,” says Stein.
Thalassemia is a genetic blood disorder that causes your body to have less hemoglobin than normal.
But RDW isn’t a standalone way to diagnose disease, Stein says.
“Clinicians would never make those judgments alone based on RDW – they would always follow the test with directed analysis, such as iron studies or hemoglobin electrophoresis,” he says.
What level of RDW is dangerous?
Doctors don’t consider RDW results themselves dangerous. They’re just one piece of the puzzle when looking at your circulatory health.
“There is no such thing as a concerning value,” says Stein. “This is one of many parameters in a CBC that could help a physician in their approach to identifying the cause of anemia.”
Takeaways
A red blood cell distribution width (RDW) test is part of the complete blood count (CBC) test doctors use to look at your overall blood health. RDW measures the variation in red blood cell (RBC) size and volume. RDW can help diagnose anemia and conditions related to anemia such as iron deficiency, vitamin B12 deficiency, or thalassemia. RDW isn’t a standalone diagnostic tool; doctors use it alongside other blood test results and other tests to piece together a diagnosis and plan for treatment.
RDW Blood Test FAQs
What does it mean if my RDW is high?
High RDW results mean the red blood cells in your blood sample were very different sizes when compared to each other. A high RDW can be a sign of anemia or related conditions.
What does it mean when your RDW is low?
Low RDW means that the red blood cells in your blood sample are very close in size and volume to each other. This is not a sign of a problem.
Is your RDW low if you have iron deficiency anemia?
If you have iron deficiency anemia, you’re more likely to have a high RDW.
Can dehydration cause high RDW?
Dehydration can cause an overall high red blood cell count. This could change your RDW, but it’s not a given.
Does high RDW cause fatigue?
A high RDW can be one sign of anemia, which is a condition where your body doesn’t make enough red blood cells. Because of the decrease in red blood cells in anemia, your tissues don’t get enough oxygen, and a common symptom is fatigue.
What cancers cause high RDW?
Studies show that there is a link between a high RDW and an increased risk of cancers such as carcinomas (lungs, breasts, colon), sarcomas (bones, muscles, blood vessels), brain tumors, lymphomas, leukemia, and multiple myeloma.