Red Blood Cell (RBC) Redistribution Width (RBC RDW) by Automated Counting
This test measures the variation in size (redistribution width) of your red blood cells (RBCs) using an automated blood cell counter.
["RBC RDW helps doctors assess how evenly sized your red blood cells are, which can indicate certain blood disorders or health conditions."]
Key facts at a glance
- LOINC
- 21000-5
- Short code
- RDW RBC Auto
- Synonyms
- RDW RBC Auto
- Category
- chemistry
- Sample
- serum
- Container
- Red-top tube (SST)
- Units
- N/A
- Ref low
- 4.2
- Ref high
- 5.9
- Critical range
- 2 N/A – 7 N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- Method
- automated-count
- CPT
- 85041
- SNOMED
- 900000000000003001
- Specimen volume
- 5 mL
- Stability
- Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
- Hemolysis max
- 500
- Lipemia max
- 1000
- Icterus max
- 500
- TAT
- 4–24 hours
- Order
- routine
What Does RBC RDW Measure?
RBC RDW evaluates the spread in size among your red blood cells. Normally, RBCs are relatively uniform in size, but certain conditions—such as anemia, iron deficiency, or other blood disorders—can cause a wider range of sizes. This test helps identify if your RBCs are consistently similar in size or if there is significant variation.
Why Might Your Doctor Order This Test?
Your doctor may request an RBC RDW test if they suspect conditions like anemia (low red blood cell count), iron deficiency, or other blood-related disorders. It can also be part of routine blood work to monitor chronic conditions or check for subtle changes in your blood health.
How Is This Test Performed?
This test uses a small sample of your blood, typically collected via a simple vein puncture. The blood is analyzed automatically by a machine that measures the size and shape of each red blood cell. No special preparation is needed, but fasting is not required.
What Are Common Uses of RBC RDW?
RBC RDW is often used alongside other blood tests to diagnose or monitor conditions such as megaloblastic anemia (caused by vitamin deficiencies like B12 or folate), thalassemia, or other inherited blood disorders. It can also help guide treatment decisions for iron deficiency or other blood-related issues.
Reference range
Adult males: 4.2-5.9; Females: 3.8-5.1 x 10^12/LN/A
No special preparation required. Stay well hydrated.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
An elevated RBC RDW may suggest underlying issues like anemia, nutritional deficiencies, or chronic diseases. A normal RBC RDW does not rule out all blood disorders but can help narrow down potential causes when combined with other test results. Always discuss your results with your healthcare provider for personalized interpretation.
Short name
RBC RDW Auto
Frequently asked questions
What Does an Elevated RBC RDW Mean?
What Does a Normal RBC RDW Indicate?
Does RBC RDW Require Special Preparation?
Sources
- 1
- 2Lab Tests OnlineSource
- 3
Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.
This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.
This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.
If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.
Medically reviewed by WellAlly Editorial Team Last reviewed August 17, 2026. · Read our editorial policy