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Red Blood Cell Volume Distribution Width (RDW) - Calculated

Published: August 9, 2026Updated: August 17, 2026

This test measures the variation in size of your red blood cells (RBCs). It helps your healthcare provider assess potential blood disorders, such as anemia or iron deficiency.

Quick summary

["RDW can indicate whether your red blood cells are too large or too small, which may signal underlying health concerns."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 17, 2026Editorial policy

Key facts at a glance

LOINC
111545-0
Short code
Erythrocyte vol deviation Patient Calc
Synonyms
Erythrocyte vol deviation Patient Calc
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
calculated
CPT
85041
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 Is RDW-Calc Used For?

RDW-Calc is primarily used to diagnose and monitor certain types of anemia. A higher RDW may suggest conditions like vitamin deficiencies, chronic diseases, or blood disorders. It can also help guide treatment decisions by showing how well your body is producing healthy red blood cells.

How Is This Test Performed?

This test involves a simple blood draw from a vein in your arm. Your blood is analyzed to measure the range of sizes of your red blood cells. The results are then calculated to determine the RDW.

What Might Affect RDW-Calc Results?

Several factors can influence RDW levels, including certain medications, chronic illnesses, pregnancy, and nutritional deficiencies (like iron, vitamin B12, or folate). Recent blood transfusions or certain medical conditions may also impact these results.

Interpreting Your RDW-Calc Results

Your RDW-Calc result is compared to a reference range provided by your healthcare provider. While exact ranges can vary, higher values may suggest red blood cells are too varied in size, which can be linked to anemia or other blood disorders. Lower values might indicate a more uniform cell size but could also be seen in certain genetic conditions.

Reference range

Adult males: 4.2-5.9; Females: 3.8-5.1 x 10^12/LN/A

Specimen: serum

No special preparation usually required. Stay well hydrated. Inform your healthcare provider of all medications and supplements you are taking.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance of RDW-Calc

Short name

RDW-Calc

Frequently asked questions

Why do I need this test?
Your healthcare provider may order an RDW-Calc test if you have symptoms like fatigue, pale skin, or shortness of breath, which can be signs of anemia. It may also be used to monitor conditions like pernicious anemia, thalassemia, or other blood-related disorders. Additionally, it can help assess how well your body is responding to treatments for these conditions.
What does an abnormal RDW-Calc mean?
An elevated RDW-Calc often points to conditions where red blood cells are not uniform in size, such as iron deficiency anemia, vitamin deficiencies, or chronic diseases. A lower RDW-Calc might suggest conditions like microcytic anemia or certain genetic disorders. Always discuss your results with your healthcare provider, as they can provide personalized insights based on your medical history and other test results.
Can I do anything to prepare for this test?
No special preparation is needed for an RDW-Calc test. However, inform your healthcare provider about any medications you are taking, recent blood donations, or any medical conditions you have. Avoiding alcohol and strenuous activity before the test can help ensure accurate results.
  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 17, 2026. · Read our editorial policy