What Level Of RDW Is Dangerous? Causes, Risks and Next Steps

RDW, or Red Cell Distribution Width, measures how much your red blood cells vary in size. An abnormal RDW does not diagnose a specific condition, but it can help identify issues such as anemia, nutritional deficiencies, inflammation or chronic disease.

If your CBC shows a high or low RDW, understanding what the result means and whether additional testing is needed can help you take the right next steps for your health.

What Level Of RDW Is Dangerous?

RDW or Red Cell Distribution Width, measures how much your red blood cells vary in size. A typical RDW normal range is about 11.5% to 14.5%, though some labs use slightly different ranges. RDW above 15% to 16% may suggest anemia, nutritional deficiency, inflammation or another health issue. Very high, very low or persistent abnormal RDW should be reviewed by a healthcare provider.

In general, RDW above 16% is more concerning, especially if your hemoglobin, hematocrit or MCV is also abnormal. RDW above 18% may need prompt medical follow-up, particularly if you have fatigue, shortness of breath, dizziness, chest discomfort or signs of bleeding.

The key point: RDW is a clue, not a diagnosis.

What Is RDW In A Blood Test?

RDW stands for Red Cell Distribution Width. It is part of a complete blood count also called a CBC.

Your red blood cells carry oxygen throughout your body. Normally, these cells are fairly similar in size. RDW measures how much their size varies.

A higher RDW means your red blood cells are more uneven in size. This can happen when your body is making new red blood cells that are smaller or larger than usual.

Doctors use the RDW blood test to help evaluate:

Iron deficiency anemia
Vitamin B12 deficiency
Folate deficiency
Blood loss
Chronic inflammation
Liver or kidney disease
Bone marrow problems
Mixed types of anemia

RDW is most useful when it is interpreted with other blood test results, especially MCV, hemoglobin and hematocrit.

RDW Normal Range

For most adults, the typical RDW range is:

11.5% to 14.5%

However, your lab may list a slightly different reference range. Some labs may use 12% to 15% as normal. That is why you should always compare your result with the range shown on your own blood test report.

RDW Normal Range By Age

RDW can vary by age, lab method and overall health. Newborns and children may have different reference ranges than adults. Older adults may also have slightly higher RDW because chronic illness, inflammation, kidney disease, nutritional deficiencies and medication use become more common with age.

RDW Level	General Meaning	What It May Suggest
Below lab range	Low RDW	Usually less concerning by itself; interpret with other CBC values
11.5%–14.5%	Normal RDW	Red blood cells are fairly similar in size
14.6%–15.9%	Mildly high RDW	Possible early deficiency, anemia, blood loss or lab variation
16%–18%	High RDW	More likely anemia, iron deficiency, B12/folate deficiency, inflammation or chronic disease
Above 18%	Significantly high RDW	Needs medical review, especially with symptoms or abnormal hemoglobin

What Causes RDW To Be High?

A high RDW means your red blood cells vary more than expected in size. This can happen for several reasons.

Iron Deficiency Anemia

Iron deficiency is one of the most common causes of RDW high blood test results. It often causes smaller red blood cells, which can increase RDW.

Possible causes include:

Heavy menstrual bleeding
Low iron intake
Digestive bleeding
Pregnancy
Poor iron absorption
Frequent blood donation

Vitamin B12 Or Folate Deficiency

Vitamin B12 and folate help your body make healthy red blood cells. When either is low, red blood cells may become larger than normal, causing elevated RDW.

Symptoms may include fatigue, weakness, numbness, tingling, memory issues or balance problems.

Blood Loss

Recent or ongoing blood loss can raise RDW as your body produces new red blood cells. This may happen with heavy periods, surgery, injury, gastrointestinal bleeding or hidden blood in the stool.

Chronic Inflammation Or Infection

Long-term inflammation can affect how your body produces red blood cells. RDW may be high in people with chronic infections, autoimmune disease, kidney disease, inflammatory bowel disease or other ongoing illnesses.

Liver, Kidney Or Thyroid Disease

These conditions can affect red blood cell production, nutrient balance and red blood cell survival. RDW may be abnormal along with other lab changes.

Bone Marrow Disorders

Less commonly, high RDW may be linked to bone marrow disorders or blood cell production problems. This is more concerning if RDW is high along with abnormal white blood cells, low platelets, easy bruising, frequent infections, fevers, night sweats or unexplained weight loss.

High RDW Symptoms

High RDW itself usually does not cause symptoms. Symptoms usually come from the underlying condition causing it.

Possible high RDW symptoms include:

Fatigue
Weakness
Dizziness
Shortness of breath
Pale skin
Rapid heartbeat
Chest discomfort
Headaches
Cold hands and feet
Trouble concentrating
Numbness or tingling if B12 deficiency is involved

Seek urgent care if you have chest pain, fainting, severe shortness of breath, black or bloody stools or severe weakness.

Is High RDW A Sign Of Cancer?

Many patients worry about this question: is high RDW a sign of cancer?

High RDW does not automatically mean cancer. In fact, elevated RDW is more commonly related to iron deficiency, vitamin B12 deficiency, folate deficiency, anemia, inflammation or blood loss.

However, cancer or bone marrow disorders can sometimes affect blood counts. Doctors become more concerned when high RDW appears with other abnormal results, such as low hemoglobin, abnormal white blood cells, low platelets, unexplained weight loss, night sweats, persistent fever or enlarged lymph nodes.

If your RDW is high, the best next step is to review your full CBC with a healthcare provider.

What Causes Low RDW?

A low RDW blood test result means your red blood cells are very similar in size. Low RDW is usually less clinically important than high RDW.

In many cases, low RDW is not dangerous if the rest of your CBC is normal.

Possible explanations include:

Normal lab variation
Uniformly small red blood cells
Uniformly large red blood cells
Lab-specific reference differences
Chronic conditions when other blood values are also abnormal

Is Low RDW Bad?

Usually, low RDW is not bad by itself. But if low RDW appears with low hemoglobin, abnormal MCV, fatigue, weakness or shortness of breath, your provider should interpret it with the rest of your blood work.

Like high RDW, low RDW is not usually meaningful alone.

How RDW Works With MCV, Hemoglobin And Hematocrit?

RDW becomes more helpful when your doctor compares it with other CBC values.

MCV measures the average size of your red blood cells.
Hemoglobin measures the oxygen-carrying protein in red blood cells.
Hematocrit measures how much of your blood is made up of red blood cells.

Here is how doctors may interpret common patterns:

Pattern	Possible Meaning
High RDW + Low MCV	Often seen with iron deficiency anemia
High RDW + High MCV	May suggest B12 deficiency, folate deficiency, liver disease or alcohol-related changes
High RDW + Normal MCV	May occur early in iron, B12 or folate deficiency
Normal RDW + Low MCV	May suggest thalassemia trait or long-standing small red blood cells
High RDW + Low Hemoglobin	Anemia is likely and should be evaluated

This is why you should not try to diagnose yourself from RDW alone.

Health Risks Associated With Abnormal RDW

Abnormal RDW can point to conditions that may need treatment.

Possible risks include:

Untreated anemia
Severe fatigue and weakness
Shortness of breath
Nutritional deficiencies
Nerve problems from untreated B12 deficiency
Hidden blood loss
Chronic inflammation
Increased strain on the heart in people with anemia or heart disease
Possible bone marrow or blood disorders in rare cases

Some studies have linked elevated RDW with heart disease, stroke, kidney disease and chronic illness. However, RDW is not used alone to predict these conditions. It is one piece of a larger health picture.

Next Steps After Abnormal RDW

If your RDW is abnormal, do not panic. Follow these steps:

Review the full CBC

Look at RDW with hemoglobin, hematocrit, MCV, RBC count, white blood cells and platelets.

Compare your result with your lab’s range

A slightly high RDW may be less concerning than a rising or significantly elevated result.

Talk to a healthcare provider

A primary care provider can help connect the dots between your RDW result, symptoms, medical history, diet, medications and other CBC values. If your RDW is high or low, we offer primary care services to help review abnormal blood work, identify possible causes and recommend the right follow-up testing.

Ask about follow-up tests

Depending on your results, your provider may also recommend health screenings to look for related issues such as anemia, nutritional deficiencies, diabetes, cholesterol problems, thyroid imbalance, kidney disease or other chronic conditions that may affect your overall health.

Treat the cause

Treatment may involve iron, B12, folate, dietary changes, managing chronic disease, evaluating blood loss or referral to a specialist.

At Manhattan Primary Care, our team can help review abnormal blood test results, order appropriate lab testing and guide your next steps.

Conclusion

RDW is an important part of a CBC, but it should not be interpreted alone. A normal RDW is usually around 11.5% to 14.5%, while RDW above 15% to 16% may need medical review.

If your RDW is high, low, or paired with symptoms, schedule a visit with a healthcare provider. Manhattan Primary Care can help you understand your blood work, identify possible causes, and choose the right next steps.

Frequently Asked Questions

Can RDW change quickly?

RDW can change over time, but usually not overnight. It may shift during anemia recovery, after blood loss or after starting iron, B12 or folate treatment.

Can dehydration affect RDW results?

Dehydration usually does not directly raise RDW, but it can affect other blood values. Your doctor may repeat the CBC if results seem unusual.

Does a normal RDW rule out anemia?

No. Some types of anemia can occur with normal RDW. Hemoglobin, hematocrit, MCV and iron studies are also needed.

Can RDW be high after taking iron supplements?

Yes. RDW may stay high temporarily as your body makes new red blood cells during recovery.

Is RDW different from RBC count?

Yes. RDW measures red blood cell size variation. RBC count measures the number of red blood cells.

Should I repeat my RDW test if it is abnormal?

Often, yes. A repeat CBC can confirm whether RDW is persistent, improving or related to a temporary issue.

Sources

MEDEX – What Level of RDW Is Dangerous? Understanding High RDW in Blood Tests
Manhattan Medical Arts – Understanding The Risks: What Level of RDW is Dangerous?
Your Health Magazine – Is Your RDW Level in the Danger Zone? Discover the Risks and What It Means for Your Health
Cardiology Care NYC – What Level of RDW Is Dangerous? Normal vs High RDW Explained
Ambar Lab – High RDW: what does it mean?
Health Line – Red Cell Distribution Width (RDW) Test

Disclaimer

This blog is for informational & educational purposes only and does not intend to substitute any professional medical advice or consultation. For any health-related concerns, please consult with your physician, or call 911.