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Red Blood Cell Distribution Width (RDW) - Standard

Published: August 9, 2026Updated: August 16, 2026

This test measures the variation in size of your red blood cells. It helps your healthcare provider assess whether your red blood cells are consistently the same size or if some are unusually large or small.

Quick summary

["RDW helps detect anemia types and other blood disorders by checking red cell uniformity."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
14907-0
Short code
Rh Bld BPU
Synonyms
Rh Bld BPU
Category
chemistry
Sample
whole-blood
Container
Lavender-top (EDTA)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Specimen volume
3 mL
Stability
Room temperature: 4h, Refrigerated: 24h
Hemolysis max
0
Lipemia max
0
Icterus max
0
TAT
4–24 hours
Order
routine

Why This Test Matters

Your red blood cells carry oxygen throughout your body. If they’re all about the same size, your body works smoothly. If they vary widely, it could signal problems like iron deficiency, vitamin deficiencies, or other blood disorders. RDW is a simple but important part of a full blood count (CBC).

What to Expect Before the Test

You won’t need special preparation. This test is usually done on a small blood sample drawn from your arm. It’s quick and painless, similar to getting an injection. No fasting or special diet is required.

How the Test Works

A healthcare professional will take a small blood sample. The lab then analyzes the sample to measure the size range of your red blood cells. This is done using automated technology that counts and sizes each cell precisely.

What the Results Mean

Results are reported as a percentage (typically between 11.5% and 14.5%). A higher RDW means more variation in cell size, which may indicate conditions like anemia or other blood disorders. A lower RDW suggests cells are more uniform. Your doctor will interpret results in the context of your overall health and symptoms.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

RDW

Frequently asked questions

What causes RDW to change?
RDW can change due to underlying health issues like anemia (from low iron, folate, or vitamin B12), chronic diseases (such as kidney disease or liver disease), or conditions affecting bone marrow function. It can also be influenced by certain medications or infections.
Does RDW alone diagnose a condition?
No, RDW is one piece of the puzzle. It’s used alongside other test results and symptoms to help diagnose conditions like anemia. Your doctor will consider the full picture, including your medical history and other lab tests.
Can RDW be affected by diet or lifestyle?
Diet and lifestyle can indirectly influence RDW. For example, deficiencies in iron, folate, or vitamin B12—commonly linked to poor diet—can raise RDW. However, lifestyle factors like smoking or chronic stress may also play a role, though they’re not the direct cause. Always discuss your symptoms and test results with your healthcare provider.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

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.

Medical Disclaimer

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 16, 2026. · Read our editorial policy