Understanding Your RDW Test Results

📋 At a Glance

• What it is: A measurement that shows how much your red blood cells vary in size from each other
• Found in tests: Complete Blood Count (CBC), CBC with Differential
• Normal range: 11.5-14.5% for most adults (may vary slightly by lab)

If you're looking at your lab results and wondering what RDW means, you're in the right place. Think of RDW as a quality control inspector at a factory - it checks whether your red blood cells are coming out in uniform sizes or if there's too much variation. When your body is making red blood cells properly, they're all roughly the same size, like cookies from the same cookie cutter. Let's break down what your numbers mean in plain English.

Why Is RDW Tested?

Your doctor includes RDW as part of your Complete Blood Count (CBC) because it provides valuable clues about what's happening inside your body. This test is often ordered during routine check-ups, when you're feeling unusually tired, or if your doctor suspects you might have anemia or another blood condition.

RDW becomes particularly important when combined with other blood tests. While your hemoglobin might tell us if you have enough red blood cells, RDW tells us about the quality and consistency of those cells. It's like the difference between knowing you have enough workers in a factory versus knowing if they're all working efficiently.

This measurement helps your doctor distinguish between different types of anemia, monitor how well you're responding to treatment, and sometimes catch nutritional deficiencies before they cause major symptoms. It's a simple test that provides surprisingly detailed information about your overall health.

What Does RDW Do in Your Body?

RDW doesn't actually "do" anything in your body - it's a measurement, not a substance. It stands for Red Cell Distribution Width, which is a fancy way of saying "how different are your red blood cells from each other in size?" Your bone marrow is constantly producing new red blood cells, and ideally, they should all be about 6-8 micrometers in diameter - about the width of a spider's silk thread.

When everything is working well, your bone marrow produces red blood cells like a well-oiled machine, each one nearly identical to the last. These cells live for about 120 days, carrying oxygen from your lungs to every tissue in your body. But when something disrupts this process - like a vitamin deficiency, chronic disease, or bone marrow problem - you might start producing cells of varying sizes.

The RDW percentage tells us how much variation exists. A low percentage means your cells are uniform (which is usually good), while a high percentage means there's a lot of size variation (which often signals that something needs attention). It's particularly useful when looked at alongside your MCV (mean corpuscular volume), which tells us the average size of your red blood cells.

What Do My RDW Results Mean?

Normal RDW Ranges

Note: Reference ranges may vary slightly between laboratories. Always compare your results to the range provided on your specific lab report.

What Does High RDW Mean?

Common Causes:

• Iron deficiency anemia (your body can't make proper red blood cells without enough iron)
• Vitamin B12 deficiency (causes production of abnormally large red blood cells)
• Folate deficiency (similar to B12, affects cell production)
• Mixed anemias (having more than one type of anemia simultaneously)
• Recent blood transfusion (mixing your cells with donor cells of different sizes)
• Chronic liver disease (affects how your body processes nutrients needed for blood cell production)
• Thyroid disorders (can affect red blood cell production and lifespan)

Possible Symptoms:

• Unusual fatigue or weakness
• Shortness of breath with minimal exertion
• Pale skin or nail beds
• Dizziness or lightheadedness
• Cold hands and feet
• Rapid or irregular heartbeat

When to Be Concerned: An RDW above 14.5% warrants discussion with your doctor, especially if accompanied by symptoms. Values above 20% often indicate significant anemia or other blood disorders requiring prompt evaluation. If you're experiencing chest pain, severe shortness of breath, or fainting, seek immediate medical attention regardless of your RDW level.

What Does Low RDW Mean?

Common Causes:

• Recent blood loss followed by rapid recovery
• Certain types of thalassemia (inherited blood disorders)
• Chronic disease anemia with stable production
• Some cases of aplastic anemia
• Post-splenectomy (after spleen removal)
• Certain medications affecting blood cell production
• Laboratory variation (sometimes just normal for you)

Possible Symptoms:

• Often no symptoms with low RDW alone
• Fatigue if associated with anemia
• Weakness
• Pale appearance

When to Be Concerned: Low RDW is rarely concerning on its own and is often considered a normal variant. However, if it's accompanied by other abnormal blood counts or you're experiencing symptoms, discuss with your doctor. Very low values (below 10%) combined with anemia might indicate certain genetic conditions.

What Can Affect My RDW Levels?

Factors That May Increase Levels:

• Medications: Chemotherapy drugs, antiretroviral medications, some antibiotics
• Lifestyle: Poor diet lacking iron, B12, or folate; excessive alcohol consumption; chronic stress
• Conditions: Recent infections, inflammation, pregnancy (mild increase), menstruation
• Supplements: Starting iron supplementation (temporarily, as new cells are produced)

Factors That May Decrease Levels:

• Medications: Some immunosuppressants, certain blood pressure medications
• Lifestyle: Very consistent, nutrient-rich diet; regular exercise
• Conditions: Well-controlled chronic conditions, post-transfusion state
• Time of Day: Minimal variation throughout the day

How Is RDW Related to Other Tests?

RDW is most meaningful when interpreted alongside other blood tests, particularly those in your CBC panel. It's like a piece of a puzzle that makes more sense when you see the whole picture.

Often tested alongside: MCV (mean corpuscular volume), hemoglobin, hematocrit, red blood cell count, reticulocyte count Part of: Complete Blood Count (CBC), CBC with Differential Ratio calculations: RDW-CV (coefficient of variation) and RDW-SD (standard deviation) are two ways to express the same concept Follow-up tests: Iron studies, vitamin B12, folate, thyroid function tests, hemoglobin electrophoresis

Frequently Asked Questions

How often should RDW be tested? For healthy adults, RDW is typically checked annually as part of routine CBC testing. If you have anemia or are being treated for a blood condition, your doctor may check it monthly or quarterly to monitor your response to treatment.

Can I improve my RDW levels naturally? If your high RDW is due to nutritional deficiencies, eating iron-rich foods (lean meats, leafy greens), B12 sources (fish, dairy), and folate-rich foods (legumes, fortified grains) can help. However, always work with your doctor to identify the underlying cause first.

Should I fast before an RDW test? No fasting is required for RDW testing. You can eat and drink normally before your blood draw.

How quickly can RDW levels change? RDW changes gradually over weeks to months as old red blood cells are replaced with new ones. You typically won't see significant changes in less than 4-6 weeks, even with treatment.

Next Steps After Your RDW Test

Questions to Ask Your Doctor:

• What do my RDW results mean for my overall health?
• Are my levels related to my symptoms?
• Do I need additional testing?
• Should we monitor this over time?
• Are there lifestyle changes that could help?

Download our RDW Doctor Questions Checklist

🔬 Ready to Track Your RDW Over Time?

Understanding a single RDW result is just the beginning. Our free Lab Analyzer tool helps you:

• Track how your RDW changes over time
• See how it relates to your other lab values
• Identify patterns your doctor might miss
• Get personalized insights based on your trends

[Upload Your Lab Report for Free Analysis →]