RDW Blood Test: What It Is, Why It’s Done, and How to Read the Results

When it comes to your health, blood tests often tell stories that your body cannot say aloud. One such test is the RDW blood test. Though it sounds technical, understanding it can really help you know what is happening inside your body.

RDW stands for Red Cell Distribution Width. It checks how much your red blood cells differ in size and shape. This small difference can actually show bigger problems like anaemia, heart disease, and even cancer.

Also, experts regularly check RDW because even if your haemoglobin looks okay, RDW might still catch a hidden issue. Therefore, doctors often look at RDW values along with other blood parameters.

Now, let’s break down everything you need to know.

What Is RDW in a Blood Test?

RDW stands for Red Cell Distribution Width. It’s part of a Complete Blood Count (CBC) and describes how much your red blood cells (RBCs) vary in size.

• Low variation = most RBCs are about the same size → RDW is normal

• High variation = mixture of smaller and larger RBCs → RDW is high (anisocytosis)

Why does that matter? Red blood cells carry oxygen through hemoglobin. When RBCs are being produced abnormally (or replaced quickly), you may see a wider spread of sizes, and RDW can become an early clue. 

Now, here’s a detail many skip, but it can explain why your RDW looks “off” compared to someone else’s report.

RDW-CV vs RDW-SD (Why Some Reports Use Different RDW Numbers)

You might see RDW reported as:

• RDW-CV (a percentage, most common), or

• RDW-SD (a number in fL, less common)

Key point: RDW-CV can be influenced by the MCV (average RBC size), especially in microcytosis (small cells). RDW-SD is considered a more direct measure of size variation because it’s less affected by MCV. 

RDW-CV is % and can be distorted when MCV is low; RDW-SD is a more “absolute” size-variation measure.

Why Doctors Ask for RDW Test: Real Reasons

You might wonder why a doctor would even ask for an RDW test. It is not only about anaemia, but so much more.

Here’s when the RDW test becomes necessary:

• Symptoms like tiredness, breathing trouble, dizziness, and pale skin.

• Suspected vitamin and mineral deficiencies, like iron, folate, or B12.

• Chronic infections or autoimmune conditions like HIV/AIDS.

• Possible genetic disorders like Thalassemia or Sickle Cell Disease.

• Liver or kidney diseases.

• Heart problems.

• Cancer risk evaluation.

Many times, RDW helps the doctor spot diseases early before they become worse. Even without obvious symptoms, a small RDW rise can suggest hidden dangers.

When Should You Go for an RDW Blood Test? Watch These Signs

Sometimes the body gives small warnings. If you have any of these symptoms, your doctor might suggest RDW testing:

• Getting tired easily without hard work.

• Feeling cold even in normal weather.

• Regular headaches without other causes.

• Pale or dry skin.

• Shortness of breath while climbing stairs.

• Feeling dizzy after standing up.

Moreover, if you have conditions like diabetes, Crohn’s disease, or long-term infections, RDW monitoring becomes a regular part of blood health checkups.

Sometimes, RDW is checked before and after surgeries, too, especially when there is major blood loss expected.

So when should you personally care about RDW? Often, it starts with symptoms that seem “normal” but keep repeating.

How the RDW Test is Performed: Simple and Fast

Getting an RDW test is very simple. No big preparation needed.

• No fasting needed unless combined with other tests.

• A healthcare professional will take a small sample of blood using a thin needle.

• Only a slight pinch or scratch will be felt.

The whole process finishes in under 5 minutes. Results usually come within a day or two.

What RDW Blood Test Results Mean: Normal, High or Low

Here comes the part that many people wait for: What do the RDW results actually mean?

Normal RDW Range

Normally, RDW stays between 12% to 15%. Slight variations happen depending on the lab. If your result lies inside this range, most of your red cells are uniform in size.

Yet, don’t celebrate too early. Some types of anaemia, like thalassemia minor, still show normal RDW but cause health troubles. That’s why doctors also cross-check MCV and MCH results.

High RDW Levels

A high RDW number is a warning sign.
It may point towards:

• Iron-deficiency anaemia

• Vitamin B12 or Folate deficiency

• Sickle cell disease

• Liver disease

• Heart failure

• Cancer spread

In fact, a 2014 study showed people with very high RDW were 71% more likely to suffer a stroke than those with normal RDW.

Low RDW Levels

Low RDW is not often a worry. It simply means very little size variation among red blood cells. No specific disease directly connects with low RDW. But, it’s still part of the full blood picture your doctor will study.

RDW Test Combined With Other Red Blood Cell Tests

Doctors never look at RDW alone. They also check other red cell indices:

By combining RDW with MCV, doctors can narrow down the diagnosis much faster.

For example:

• High RDW + Low MCV = Iron deficiency anaemia.

• High RDW + High MCV = Vitamin B12 or Folate deficiency.

This cross-matching method saves time and improves the accuracy of treatment planning.

Other Medical Conditions Linked with High RDW

Besides anaemia, many other conditions can raise RDW.
Let’s see a few examples:

1. Heart Disease

High RDW is closely linked to heart attacks, strokes, and heart failures. Patients with existing blood pressure problems need regular RDW checks.

2. Liver and Kidney Diseases

Chronic liver or kidney trouble affects red blood cell production. This leads to bigger differences in cell size, pushing RDW upwards.

3. Cancer

Studies show high RDW values in people with blood cancers, lung cancer, and colon cancer. It can also hint at how aggressive a cancer might become.

4. Chronic Infections

Diseases like HIV or autoimmune disorders cause internal inflammation. This, in turn, changes the production of red blood cells, raising RDW slowly over time.

When to See a Doctor Urgently

RDW alone is rarely an emergency, but seek medical advice promptly if you have:

• Chest pain, fainting, severe shortness of breath

• Rapid heartbeat with weakness

• Black/tarry stools, vomiting blood, or heavy, uncontrolled bleeding

• Severe fatigue that worsens quickly

• New neurological symptoms (face droop, speech trouble, one-sided weakness)

For non-urgent situations (most cases), book a routine appointment and bring your full CBC report.

How to Get Ready for an RDW Test?

Though the test itself is very easy, here are some tips:

• Wear short sleeves for easy access to your veins.

• Drink enough water, as it plumps up your veins and makes blood drawing simpler.

• If fasting is needed for other tests, follow the doctor’s orders.

• Inform if you are taking medicines like blood thinners or iron supplements.

Final Thoughts 

The RDW blood test is a small line on your CBC, but it can be a powerful clue. In simple terms, RDW tells you how mixed your red blood cell sizes are. A high RDW often points toward nutrient deficiency patterns, recent blood turnover, or inflammation, while a low RDW is usually not concerning. The most accurate interpretation comes from reading RDW together with MCV, hemoglobin, and sometimes iron/B12 tests. 

If your RDW is abnormal and you have symptoms like fatigue, breathlessness, dizziness, or paleness, talk to your doctor, because the goal isn’t to “treat RDW,” it’s to treat what’s causing the change.

Disclaimer: This article is for educational purposes only and does not replace medical advice. Always consult a qualified healthcare professional for interpretation of lab results and treatment decisions.