RDW Blood Test: Purpose, Procedure and Result Interpretation

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

A red blood cell distribution width, or RDW, blood test measures the variation in size and volume of your red blood cells. Red blood cells carry oxygen throughout your body, and their uniform size helps them do their job efficiently.

In healthy people, these cells are all the same size, but abnormal variation indicates that there may be health issues such as anaemia or nutrient deficiencies such as vitamin B12 or iron.

RDW is part of a complete blood count (CBC) and is often combined with other tests, such as mean corpuscular volume (MCV), to determine the type and cause of anaemia.

What is it used for?

The RDW test is used for:

• Diagnosing anaemia

  • Your RDW results assist in identifying abnormal differences in your red blood cells, which point to various forms of anaemia.

• Distinguishing between the different types of anaemia

  • The test can help your doctor distinguish between iron-deficiency anaemia, vitamin B12 deficiency anaemia, and sickle cell anaemia, as each affects your red blood cells differently.

• Detecting other health conditions

  • Abnormal RDW values in your blood may signal:

    • Diabetes mellitus

    • Heart disease

    • Liver disease

    • Kidney disease

    • Certain types of cancer

    • Thalassaemia (inherited blood disorders that can cause severe anaemia)

• Monitoring your treatment

  • If you're receiving treatment for blood-related conditions, RDW tests can help track how well your treatment is working.

Your RDW results, together with other blood tests, give your doctor valuable information about your health and guide any proper treatment you might need.

Why do I need an RDW test?

Your doctor may recommend an RDW test if you:

• Have symptoms of anaemia, such as fatigue, weakness, pale skin, dizziness, or shortness of breath.

• Are undergoing a routine health checkup, as the RDW test is part of a complete blood count.

• Have a known blood disorder that requires regular monitoring.

• May have vitamin deficiencies or chronic diseases, as RDW levels can help identify these conditions.

If you’re experiencing any of the symptoms of anaemia mentioned above, request an appointment with Thomson Medical. Our specialists can help diagnose your condition, including performing an RDW blood test to determine the underlying causes and providing personalised care.

How does an RDW blood test work?

An RDW blood test involves a simple blood draw. A healthcare professional will take a small sample of blood from your arm

Your blood sample is then sent to a lab where a specialist examines your red blood cells. They check the cells under a microscope to see if they're the same size.

The small amount of blood they test serves as a representative sample. This sample helps doctors understand if there are variations in red blood cell size throughout your entire body.

Will I need to do anything to prepare for the test?

You usually don't need any special preparation for an RDW test. It is a straightforward blood test that doesn't require you to change your normal routine.

However, if your RDW test is part of a more comprehensive blood panel—such as a CBC or a metabolic panel—your doctor might ask you to fast (not eat or drink anything except water) for a few hours before the test. This step helps ensure the most accurate results for other tests in the panel.

If you're unsure about preparation instructions, it's always best to check with your doctor or the testing facility before your appointment.

What should I expect during an RDW blood test?

The test only takes a few minutes and follows the same steps as any simple blood draw:

• Your healthcare provider will find the best vein, usually in the bend of your arm or on the back of your hand.

• They'll clean the area with an antiseptic wipe.

• An elastic band will be placed above the spot to make your vein easier to see and access.

• The provider will insert the needle to take the blood sample. You might feel a quick pinch or sting when this happens.

• Your blood will be drawn into a tube, which typically takes just a few seconds.

• The needle will be removed, a bandage will be applied, and your sample will be sent to the laboratory for analysis.

What do the results mean?

The RDW value is interpreted alongside other blood test results to find out what might be causing any problems. These complementary tests include:

• Hemoglobin levels

• Mean corpuscular volume (MCV)

• Red blood cell count

Your result implies certain things about your health, depending on whether the value is normal, high, or low:

Normal RDW

A normal RDW result indicates that your red blood cells are uniform in size. Typically, RDW falls between 12% and 15%, but this normal range may vary depending on the laboratory. Your test report will include the specific reference range for comparison.

However, a normal RDW doesn’t rule out underlying conditions. Some types of anaemia may show normal RDW levels but abnormal values in other red blood cell indices.

High RDW

If your RDW is high, your red blood cells vary more in size than they should. This might point to:

• Anaemia (low red blood cells)

• Nutrient deficiencies (iron, vitamin B12, or folate)

• Chronic illness

• Bone marrow disorders

Low RDW

A low RDW is rare and usually not considered clinically significant.

Your doctor will look at your complete results to get a complete picture of your health and determine if any treatment is needed.

Are there any risks to the test?

The RDW test is a routine blood test with very few risks. You might experience:

• Slight pain or bruising at the spot where the needle was inserted.

• Slight dizziness or fainting in rare cases.

• Infection at the injection site, though this is very rare.

These side effects are usually minor and go away quickly.

FAQ

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

An RDW blood test is actually a component of a CBC, not a separate test. The main differences are:

• A CBC is a comprehensive panel that measures many different components of your blood, including red blood cells, white blood cells, platelets, haemoglobin, and haematocrit, along with RDW.

• The RDW specifically measures the variation in the size of your red blood cells.

• When you get a CBC, you automatically receive RDW results as part of the overall report.

• Your doctor might order a standalone CBC for general health screening, while they would look specifically at the RDW component when identifying certain conditions like anaemia or other blood-related disorders.

How is RDW calculated?

By dividing the standard deviation (SD) of red blood cell sizes by the mean corpuscular volume (MCV), your doctor can calculate RDW as a percentage:

RDW (%) = [Standard Deviation of RBC Size / Mean Corpuscular Volume (MCV)] × 100

The higher the percentage, the more variation exists between your red blood cells. A normal RDW result is typically somewhere between 12% and 15%, though this range may vary depending on the laboratory.

RDW can also be reported as RDW-CV (%) or RDW-SD (fL) depending on how the variation is measured.

What does it mean if RDW is high?

A high RDW does not indicate a disease by itself but helps diagnose certain conditions. It means there is a greater variation in your red blood cell sizes, which may suggest:

• Iron-deficiency anaemia

• Vitamin B12 or folate deficiency anaemia

• Chronic liver disease

• Chronic inflammation or infection

• Bone marrow disorders (e.g., myelodysplastic syndrome)

• Cardiovascular disease risk

What does it mean when your RDW is low?

A low RDW is rare and usually not considered clinically significant. However, in some cases, it may indicate a uniform size of your red blood cells (which is not necessarily abnormal) or thalassaemia minor, a mild inherited blood disorder.

How can you lower a high RDW?

Lowering RDW depends on the underlying cause:

• If due to nutrient deficiencies

  • Increase iron-rich foods (e.g., red meat, spinach, lentils).

  • Consume vitamin B12 sources (e.g., eggs, fish, dairy).

  • Take folate-rich foods (e.g., leafy greens, citrus fruits).

  • Consider supplements if prescribed by your doctor.

• If due to chronic disease

  • Manage the underlying condition (e.g., liver disease, kidney disease, or infections).

• If due to bone marrow disorders

  • A haematologist may need to evaluate further and suggest appropriate treatment.

• Adopting a healthy lifestyle

  • Reduce alcohol consumption.

  • Stay hydrated.

  • Engage in regular physical activity.

Can stress cause high RDW?

Yes, chronic stress may contribute to a high RDW by:

• Increasing inflammation in your body.

• Affecting nutrient absorption (leading to vitamin deficiencies).

• Altering hormonal balance, which may impact red blood cell production.

The information provided is intended for general guidance only and should not be considered medical advice. For personalised recommendations based on your medical conditions, request an appointment with Thomson Medical.