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Red Blood Cell Distribution Width (RDW) - Standard Deviation

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

This test measures the variation in the size of your red blood cells. Healthy red blood cells are usually similar in size, but this test checks if there’s a wider range.

Quick summary

["RDW-SD helps identify conditions affecting red blood cell size uniformity."]

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

Key facts at a glance

LOINC
51006-5
Short code
DAT RBC Ql
Synonyms
DAT RBC Ql
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
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 RDW-SD Measure?

RDW-SD quantifies the standard deviation of red blood cell volume, reflecting how much they differ in size. A higher value suggests more variability, which can signal underlying blood disorders or nutritional deficiencies.

Why Might Your Doctor Order This Test?

Your doctor may request RDW-SD to diagnose or monitor conditions like anemia (e.g., iron deficiency), vitamin deficiencies (e.g., B12 or folate), or other blood disorders. It’s often used alongside other blood tests for a clearer picture.

Common Conditions Linked to RDW-SD Results

Elevated RDW-SD is commonly seen in anemia, chronic diseases, or during recovery from illness. Lower-than-expected values are less typical but may occur in rare cases of microcytic anemia or specific genetic disorders.

How Is This Test Performed?

This test is done on a small blood sample drawn from your arm. The lab analyzes the cells under a microscope or using automated technology to measure their sizes and calculate the standard deviation.

Reference range

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

Specimen: serum

No special preparation required. Stay well hydrated.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance: RDW-SD’s Role in Diagnosis

Short name

RDW-SD

Frequently asked questions

Is RDW-SD painful or uncomfortable?
No, the blood draw is quick and usually causes minor discomfort. Most people feel only a slight pinch or pressure. If you’re nervous, let your doctor know—they can use a smaller needle or apply a topical numbing cream.
How long will it take to get my results?
Results are typically available within 1–3 business days, depending on your lab’s processing time. Your doctor will discuss them with you during your follow-up visit.
Can I eat or drink anything before this test?
No special preparation is needed for RDW-SD. However, if you’re also having other blood tests (like fasting glucose), follow those specific instructions separately.
  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