RDW-SD (Red Cell Distribution Width) in fL

Whole Blood

Optimal Result: 39 - 46 fl.

CBC Blood Test Explained: Results, Normal Ranges & How to Read a Complete Blood Count Labcorp: What It Is, What It Tests, and How to Get Your Lab Results

From Healthmatters.io Team

Lab Results Explained and Tracked

RDW-SD Blood Test: High vs Low, Normal Range & What It Means

Reviewed by HealthMatters Editorial Team · Last updated March 23 2026

Key Takeaways

RDW-SD measures variation in red blood cell size
Normal range: ~39–46 fL
High RDW-SD may suggest anemia or nutrient deficiency
Low RDW-SD is usually not clinically significant
RDW-SD must be interpreted with hemoglobin and MCV

What does RDW-SD mean on a blood test?

Most people with a slightly high or slightly low RDW-SD result do not have a serious condition. Mild changes are common and often temporary, especially with minor illness, recovery from infection, or nutrient shifts. Doctors interpret RDW-SD together with hemoglobin, MCV, iron levels, and the rest of your complete blood count (CBC).

RDW-SD (Red Cell Distribution Width – Standard Deviation) measures how much your red blood cells vary in size. It helps clinicians understand whether red blood cells are being produced normally and can provide early clues about anemia, nutrient deficiencies, inflammation, or recovery from illness.

Unlike RDW-CV (which is a percentage), RDW-SD is measured in femtoliters (fL) and reflects the actual width of the red blood cell size distribution.

A higher RDW-SD means there is more variation in red blood cell size.
A lower RDW-SD means your red blood cells are more uniform in size.

On its own, RDW-SD does not diagnose a condition. It is interpreted alongside hemoglobin, MCV, iron levels, and other CBC markers.

Small variations in RDW-SD are very common and are usually not dangerous when the rest of your CBC is normal.

What is the normal range for RDW-SD?

Typical reference range for adults:

RDW-SD: ~39–46 fL

Ranges vary slightly by laboratory and age.

Values just outside the range are common and not always clinically significant. RDW-SD is most meaningful when interpreted with other red blood cell markers.

RDW-SD Levels: Quick Reference

RDW-SD Level	Interpretation	Common Patterns
Below 39 fL	Low	Usually not clinically significant; indicates uniform red blood cell size
39–46 fL	Normal	Typical variation in red blood cell size
47–49 fL	Mildly High	May indicate early iron deficiency or nutrient imbalance
≥50 fL	Clearly Elevated	Often associated with anemia, vitamin B12 deficiency, or mixed red blood cell populations

Note: Always compare your result with your lab’s specific reference range, as cutoffs can vary.

What does low RDW-SD mean?

A low RDW-SD means your red blood cells are very similar in size.

Low RDW-SD is usually not dangerous and is rarely a sign of disease. In most cases, it simply means your red blood cells are uniform in size.

It may be seen when:

red blood cells are uniformly small
red blood cells are uniformly large
overall cell production is stable
other CBC values are normal

If hemoglobin, MCV, and iron levels are normal, a low RDW-SD is usually not a concern.

What does high RDW-SD mean?

A high RDW-SD indicates greater variation in red blood cell size. This is called anisocytosis.

Common causes include:

iron deficiency
vitamin B12 deficiency
folate deficiency
recent blood loss
chronic disease
bone marrow stress
recovery after anemia treatment

RDW-SD often rises early in anemia and may increase before hemoglobin drops significantly.

What level of RDW-SD is dangerous?

There is no single RDW-SD value that is dangerous on its own.

However, higher values may be more clinically significant when they occur with:

low hemoglobin
abnormal MCV
fatigue or shortness of breath
iron deficiency
chronic illness

RDW-SD above ~50 fL is more likely to reflect significant variation in red blood cell size and should be interpreted with other lab results.

Should You Worry About RDW-SD?

Most people do not need to worry about RDW-SD on its own.

Low RDW-SD: usually not clinically significant
Mildly high RDW-SD: often early or temporary
More concerning when combined with:
- low hemoglobin
- abnormal MCV
- symptoms (fatigue, weakness, shortness of breath)

RDW-SD becomes meaningful only when interpreted with the rest of your CBC.

What Do Specific RDW-SD Values Mean?

RDW-SD below 39 fL

Below the typical normal range. Usually low and not clinically significant. This indicates very uniform red blood cell size and is rarely a cause for concern.

Is RDW-SD 38 normal?

An RDW-SD of 38 fL is slightly below the typical range (39–46 fL). This is usually not clinically significant and often does not indicate any health problem.

RDW-SD 39–46 fL (normal range)

Within the typical normal range. This indicates normal variation in red blood cell size and is considered healthy.

Is RDW-SD 45 normal?

Yes. An RDW-SD of 45 fL is typically within the normal range (39–46 fL) and suggests normal variation in red blood cell size.

Is RDW-SD 47–49 fL high?

Mildly elevated. This may indicate early iron deficiency, nutrient imbalance, or developing anemia. Should be interpreted alongside hemoglobin and MCV.

Is RDW-SD 48 high?

Yes. An RDW-SD of 48 fL is mildly elevated above the typical range (39–46 fL). It may indicate early changes in red blood cell size, such as developing nutrient deficiency or mild anemia.

RDW-SD ≥50 fL

Clearly elevated. More likely associated with anemia, vitamin B12 deficiency, folate deficiency, or mixed red blood cell populations. Should be evaluated with other blood markers.

Is RDW-SD 50 high?

Yes. An RDW-SD of 50 fL is considered elevated and may be associated with anemia, vitamin deficiencies, or abnormal red blood cell production.

RDW-SD low but other labs are normal

A low RDW-SD with normal hemoglobin and MCV is usually benign.

Providers typically do not treat low RDW-SD alone. It is considered a minor variation rather than a disease marker.

What is the difference between RDW-SD and RDW-CV?

Both markers measure variation in red blood cell size:

RDW-SD

Measured in femtoliters
Absolute size variation
Not affected by MCV

RDW-CV

Percentage
Influenced by MCV
Used with anemia classification

Many clinicians consider RDW-SD a more direct measurement of size variability.

What symptoms are associated with abnormal RDW-SD?

RDW-SD itself does not cause symptoms. Symptoms come from the underlying condition, such as anemia.

Possible symptoms when RDW-SD is elevated:

fatigue
weakness
shortness of breath
dizziness
pale skin
cold hands and feet

How do doctors interpret RDW-SD?

RDW-SD is evaluated alongside:

It helps identify patterns of anemia and red blood cell production.

Key takeaway

RDW-SD measures how much your red blood cells vary in size.
A high RDW-SD may suggest anemia or nutrient deficiency.
A low RDW-SD is usually not clinically significant.

The result is most meaningful when interpreted with the rest of your CBC and your overall health.

Frequently Asked Questions About RDW-SD

What is RDW-SD in a blood test?

RDW-SD (Red Cell Distribution Width – Standard Deviation) measures how much your red blood cells vary in size. It is part of a complete blood count (CBC) and helps identify conditions like anemia, nutrient deficiencies, or abnormal red blood cell production.

What is the normal range for RDW-SD?

The typical normal range for RDW-SD is approximately 39–46 fL, though this can vary slightly depending on the laboratory.

What does high RDW-SD mean?

High RDW-SD indicates greater variation in red blood cell size. It may be associated with iron deficiency, vitamin B12 deficiency, folate deficiency, mixed anemia, or increased red blood cell turnover.

What does low RDW-SD mean?

Low RDW-SD means your red blood cells are very similar in size. In most cases, this is not clinically significant and is usually not a cause for concern.

What level of RDW-SD is dangerous?

There is no single "dangerous" level, but RDW-SD values above 50 fL are generally considered elevated and may require further evaluation, especially if other blood markers are abnormal.

Is RDW-SD 50 high?

Yes, an RDW-SD of 50 fL is considered elevated and may be associated with anemia, vitamin deficiencies, or abnormal red blood cell production.

Is RDW-SD 48 high?

An RDW-SD of 48 fL is mildly elevated. It may indicate early changes in red blood cell size, such as developing nutrient deficiency or mild anemia.

What does RDW-SD 45 mean?

An RDW-SD of 45 fL is typically within the normal range and suggests normal variation in red blood cell size.

What does RDW-SD 38 mean?

An RDW-SD of 38 fL is slightly below the typical range. This is usually not clinically significant and often does not indicate any health problem.

Is low RDW-SD dangerous?

No, low RDW-SD is generally not dangerous. It rarely indicates disease on its own and is usually considered a normal or insignificant finding.

What causes low RDW-SD?

Low RDW-SD is often due to uniform red blood cell size and typically does not have a specific medical cause. It is usually not associated with underlying disease.

Can RDW-SD be high without anemia?

Yes, RDW-SD can be elevated before anemia develops. It may indicate early iron deficiency, vitamin B12 deficiency, or other changes in red blood cell production.

What symptoms are associated with high RDW-SD?

High RDW-SD itself does not cause symptoms, but underlying conditions such as anemia may lead to fatigue, weakness, shortness of breath, dizziness, or pale skin.

What is the difference between RDW-SD and RDW-CV?

RDW-SD measures the absolute variation in red blood cell size, while RDW-CV expresses this variation as a percentage relative to the average cell size (MCV). RDW-SD is considered more direct and less influenced by MCV.

How is RDW-SD interpreted with other blood tests?

RDW-SD should be interpreted alongside hemoglobin, MCV, ferritin, vitamin B12, and folate levels. It is most useful when combined with other markers rather than viewed alone.

Lab Results Explained and Tracked

All Your Lab Results.
One Simple Dashboard.

What does it mean if your RDW-SD (Red Cell Distribution Width) in fL result is too high?

A high RDW-SD means there is more variation in the size of your red blood cells.
This is called anisocytosis.

High RDW-SD is commonly associated with anemia or nutrient deficiencies, but it does not diagnose a condition on its own.

Common causes of high RDW-SD

High RDW-SD may occur with:

Iron deficiency
One of the most common causes.

Vitamin B12 or folate deficiency

Recent blood loss

Chronic disease or inflammation

Recovery from anemia treatment

Liver disease

Bone marrow stress

RDW-SD often rises early in anemia before hemoglobin drops significantly.

RDW-SD and MCV patterns

Clinicians interpret RDW-SD together with MCV:

High RDW + low MCV
Often suggests iron deficiency.

High RDW + high MCV
May suggest B12 or folate deficiency or liver disease.

High RDW + normal MCV
Can occur in early anemia or mixed deficiencies.

When high RDW-SD is more concerning

High RDW-SD may require evaluation if it appears with:

low hemoglobin
fatigue
shortness of breath
dizziness
chronic illness

Higher values (often above ~50 fL) may reflect more significant variation in red blood cell size and should be interpreted with other labs.

Key takeaway

High RDW-SD indicates variation in red blood cell size and is often associated with anemia or nutrient deficiency. It is most meaningful when interpreted alongside hemoglobin, MCV, and iron studies.

All Your Lab Results.
One Simple Dashboard.

Import, Track, and Share Your Lab Results Easily

Import, Track, and Share Your Lab Results

Import lab results from multiple providers, track changes over time, customize your reference ranges, and get clear explanations for each result. Everything is stored securely, exportable in one organized file, and shareable with your doctor—or anyone you choose.

Cancel or upgrade anytime

What does it mean if your RDW-SD (Red Cell Distribution Width) in fL result is too low?

A low RDW-SD means your red blood cells are very similar in size.
In most cases, this is not dangerous and does not indicate a disease.

Low RDW-SD is usually considered a normal variation, especially if hemoglobin, MCV, and other CBC values are normal.

Why RDW-SD can be low

RDW-SD reflects how much red blood cell sizes vary. A lower value simply means the cells are more uniform.

This can occur when:

Normal healthy state
Many healthy people have slightly low RDW-SD with no underlying condition.

Uniform red blood cell size
If red blood cells are consistently small or consistently large, RDW-SD can appear low even when there is no serious problem.

Stable red blood cell production
When bone marrow production is steady and consistent, red blood cells tend to be similar in size.

When low RDW-SD might be evaluated further

Low RDW-SD alone is rarely concerning.
Further evaluation may be considered if it appears with:

low hemoglobin
abnormal MCV
symptoms of anemia
chronic illness

In these cases, providers look at:

iron levels
B12 and folate
inflammation markers

Symptoms related to underlying causes

Low RDW-SD itself does not cause symptoms.
If an underlying condition exists, symptoms may include:

fatigue
weakness
dizziness
shortness of breath

Most people with low RDW-SD and normal labs have no symptoms.

Key takeaway

Low RDW-SD usually reflects uniform red blood cell size and is typically not a cause for concern. It is interpreted together with the rest of your CBC and overall health.

Article Review & Sources

All our content is backed by peer-reviewed studies, academic research, and trusted medical sources. We're committed to accuracy and transparency — see our editorial policy for details.

Laboratories

Bring All Your Lab Results Together — In One Place

We accept reports from any lab, so you can easily collect and organize all your health information in one secure spot.

Pricing Table

Gather Your Lab History — and Finally Make Sense of It

Finally, Your Lab Results Organized and Clear

Choose the plan that fits you — and turn scattered lab reports into clear insights you can finally use.

Pick a plan that fits you — and turn scattered lab reports into clarity.

Personal plans

Professional plan

$79/ year

Advanced Plan

Access your lab reports, explanations, and tracking tools.

Import lab results from any provider
Track all results with visual tools
Customize your reference ranges
Export your full lab history anytime
Share results securely with anyone
Receive 5 reports entered for you
Cancel or upgrade anytime

$250/ once

Unlimited Account

Pay once, access everything—no monthly fees, no limits.

Import lab results from any provider
Track all results with visual tools
Customize your reference ranges
Export your full lab history anytime
Share results securely with anyone
Receive 10 reports entered for you
No subscriptions. No extra fees.

Professional plan

$45/ month

Pro Monthly

Designed for professionals managing their clients' lab reports

Import lab results from any provider
Track lab results for multiple clients
Customize reference ranges per client
Export lab histories and reports
Share results and reports with clients
Invite clients to their accounts
Begin with first report entered by us
Cancel or upgrade anytime

About membership

What's included in a Healthmatters membership

Import Lab Results from Any Source

Easily upload lab results from any provider, whether it's a hospital, independent lab, or home testing service. We support PDFs, scanned documents, and JPEGs from patient portals. No need to log in to multiple platforms — everything is centralized in one secure space.

Easily upload lab results from any provider — we support PDFs, scans, and images. Keep all your reports organized in one secure place.

See Your Health Timeline

Every lab result is automatically organized on a chronological timeline, giving you a complete picture of your health journey. Whether you're tracking a condition, managing treatments, or staying proactive, the timeline helps you and your doctor understand how things progress over time.

Every lab result is automatically organized on a chronological timeline, giving you a complete picture of your health journey.

Understand What Your Results Mean

Whether it's a blood test, GI panel, urinalysis, or something else, lab reports measure numerous biomarkers that reveal what's happening inside your body. Our extensive database covers over 10,000 biomarkers, providing clear, simple explanations of what each result means and how you can take action—no matter your membership level. Say goodbye to confusion and get the insights you need to better understand your health.

Our database covers over 10,000 biomarkers with clear, simple explanations—so you can finally understand your results and what to do next.

Enter Your Lab Reports Yourself — Always Free Enter Your Lab Reports — Always Free

If you'd like to handle inputting your health data into your account, you can do so with our easy-to-use data entry forms. Our user-friendly form is designed to guide you through the quick and easy submission process, making it simple to keep track of your health metrics. This is available to both Advanced plan and Unlimited plan members.

Our user-friendly form guides you through a quick, simple submission process, making it easy to enter your health metrics.

Add your results anytime with our easy entry form. It's quick, guided, and helps you stay organized — free for all members.

Visualize Your Results

View your lab data through easy-to-read graphs and tables. Quickly spot patterns, track changes, and compare results across different dates — all without digging through multiple reports. You can also select and compare graphs of specific biomarkers side-by-side to better understand how they relate and change over time.

See your lab reports in clear graphs and tables. Spot patterns, track changes, and compare results over time — all in one place.

Export Your Complete Lab History in a Single File Export Your Complete Lab History

After collecting lab results from different providers, you can download your entire history combined into a single file. Choose from PDF, Excel, or CSV formats to easily review, share, or get a second opinion—no more juggling multiple reports.

After collecting lab results from different providers, you can download your entire history combined into a single file.

Comprehensive Data Entry Service for Your Reports Data Entry Service for Your Reports

Our inclusive service handles the data entry for your lab reports. Just submit your information and we'll take care of the rest. Advanced plan members receive five reports entered for free, then $15 per report after that. Unlimited plan members receive ten report entries for free, then $15 per report after that.

Our inclusive service handles the data entry for your lab reports. Just submit your information and we'll take care of the rest.

Securely Share With Anyone You Trust

Whether you're working with a doctor, nutritionist, caregiver, or wellness coach, you can securely share your complete lab history by sending an invite link to anyone you trust. You have full control over who sees your information and for how long, ensuring your privacy and peace of mind at every step.

Share your full lab history with your doctor, nutritionist, or coach using a secure invite link.

Discover

Let Your Lab Results Tell the Full Story

Once your results are in one place, see the bigger picture — track trends over time, compare data side by side, export your full history, and share securely with anyone you trust.

Bring all your results together to compare, track progress, export your history, and share securely.

I have been using Healthmatters.io since 2021. I travel all over the world and use different doctors and health facilities. This site has allowed me to consolidate all my various test results over 14 years in one place. And every doctor that I show this to has been impressed. Because with any health professional I talk to, I can pull up historical results in seconds. It is invaluable. Even going back to the same doctor, they usually do not have the historical results from their facility in a graph format. That has been very helpful.

Anthony

Unlimited Plan Member since 2021

What fantastic service and great, easy-to-follow layouts! I love your website; it makes it so helpful to see patterns in my health data. It's truly a pleasure to use. I only wish the NHS was as organized and quick as Healthmatters.io. You've set a new standard for health tracking!

Karin

Advanced Plan Member since 2020

As a PRO member and medical practitioner, Healthmatters.io has been an invaluable tool for tracking my clients' data. The layout is intuitive, making it easy to monitor trends and spot patterns over time. The ability to customize reports and charts helps me present information clearly to my clients, improving communication and outcomes. It's streamlined my workflow, saving me time and providing insights at a glance. Highly recommended for any practitioner looking for a comprehensive and user-friendly solution to track patient labs!

Paul

Healthmatters Pro Member since 2024

We implement proven measures to keep your data safe.

At HealthMatters, we're committed to maintaining the security and confidentiality of your personal information. We've put industry-leading security standards in place to help protect against the loss, misuse, or alteration of the information under our control. We use procedural, physical, and electronic security methods designed to prevent unauthorized people from getting access to this information. Our internal code of conduct adds additional privacy protection. All data is backed up multiple times a day and encrypted using SSL certificates. See our Privacy Policy for more details.

RDW-SD (Red Cell Distribution Width) in fL

RDW-SD Blood Test: High vs Low, Normal Range & What It Means

Key Takeaways

What does RDW-SD mean on a blood test?

What is the normal range for RDW-SD?

RDW-SD Levels: Quick Reference

What does low RDW-SD mean?

What does high RDW-SD mean?

What level of RDW-SD is dangerous?

Should You Worry About RDW-SD?

What Do Specific RDW-SD Values Mean?

RDW-SD below 39 fL

Is RDW-SD 38 normal?

RDW-SD 39–46 fL (normal range)

Is RDW-SD 45 normal?

Is RDW-SD 47–49 fL high?

Is RDW-SD 48 high?

RDW-SD ≥50 fL

Is RDW-SD 50 high?

RDW-SD low but other labs are normal

What is the difference between RDW-SD and RDW-CV?

What symptoms are associated with abnormal RDW-SD?

How do doctors interpret RDW-SD?

Key takeaway

Frequently Asked Questions About RDW-SD

What is RDW-SD in a blood test?

What is the normal range for RDW-SD?

What does high RDW-SD mean?

What does low RDW-SD mean?

What level of RDW-SD is dangerous?

Is RDW-SD 50 high?

Is RDW-SD 48 high?

What does RDW-SD 45 mean?

What does RDW-SD 38 mean?

Is low RDW-SD dangerous?

What causes low RDW-SD?

Can RDW-SD be high without anemia?

What symptoms are associated with high RDW-SD?

What is the difference between RDW-SD and RDW-CV?

How is RDW-SD interpreted with other blood tests?

All Your Lab Results. One Simple Dashboard.

What does it mean if your RDW-SD (Red Cell Distribution Width) in fL result is too high?

Common causes of high RDW-SD

RDW-SD and MCV patterns

When high RDW-SD is more concerning

Key takeaway

All Your Lab Results. One Simple Dashboard.

Import, Track, and Share Your Lab Results Easily

Import, Track, and Share Your Lab Results

What does it mean if your RDW-SD (Red Cell Distribution Width) in fL result is too low?

Why RDW-SD can be low

When low RDW-SD might be evaluated further

Symptoms related to underlying causes

Key takeaway

Article Review & Sources

Bring All Your Lab Results Together — In One Place

Gather Your Lab History — and Finally Make Sense of It

Finally, Your Lab Results Organized and Clear

Advanced Plan

Unlimited Account

Pro Monthly

What's included in a Healthmatters membership

Import Lab Results from Any Source

See Your Health Timeline

Understand What Your Results Mean

Enter Your Lab Reports Yourself — Always Free Enter Your Lab Reports — Always Free

Visualize Your Results

Export Your Complete Lab History in a Single File Export Your Complete Lab History

Comprehensive Data Entry Service for Your Reports Data Entry Service for Your Reports

Securely Share With Anyone You Trust

Let Your Lab Results Tell the Full Story

Once your results are in one place, see the bigger picture — track trends over time, compare data side by side, export your full history, and share securely with anyone you trust.

Bring all your results together to compare, track progress, export your history, and share securely.

What Healthmatters Members Are Saying

Use promo code to save 10% off any plan.

We implement proven measures to keep your data safe.

All Your Lab Results.
One Simple Dashboard.

All Your Lab Results.
One Simple Dashboard.