What can an RDW test tell you?
An abnormal RDW result signals that red blood cells vary more — or less — than usual in size. A high RDW (greater variation) often suggests conditions like iron-deficiency anemia, vitamin B12 or folate deficiency, or liver disease. It can also reflect mixed anemia, where more than one cause contributes to abnormal red cell production. A low RDW is less common and typically means the red cells are relatively uniform in size, which can be normal or seen in some chronic conditions.
Because many conditions can alter RDW, clinicians always interpret it alongside MCV, which measures the average size of red blood cells:
High RDW + Low MCV: Suggests iron-deficiency anemia or thalassemia, an inherited blood disorder
High RDW + High MCV: Suggests vitamin B12 or folate deficiency
High RDW + Normal MCV: Could reflect early nutritional deficiency, liver disease, or chronic inflammation
By combining RDW with other measurements, providers narrow down potential causes and decide whether more targeted tests are needed.
What is being tested?
RDW measures the distribution of red blood cell sizes in a blood sample. Results are expressed as a percentage. A higher percentage means more size variability.
Normal RBCs have a diameter of about 6 to 8 micrometers. Variation arises when the bone marrow produces cells under stress, or when deficiencies prevent normal maturation.
Where is the RDW test typically included?
RDW is reported as part of the complete blood count (CBC), a routine panel that evaluates overall blood health. It’s also often included on an anemia panel, which helps determine the type of anemia you’re dealing with.
CBC with RDW testing is included as part of lab testing through Hers. Discover how to improve or optimize your health with clinically backed recommendations based on your results.
Who should get an RDW test?
There are no recommendations for RDW as a stand-alone screening test. It’s automatically reported whenever a CBC is ordered, such as during an annual physical or before surgery.
A provider may also order a CBC, and thus RDW, if you:
Have symptoms of anemia, such as fatigue, weakness, dizziness, shortness of breath, or pale skin
Have a chronic disease that can affect blood counts, such as kidney disease, autoimmune conditions, or cancer
Are undergoing treatment that affects your bone marrow (chemotherapy, radiation)
Are being monitored for recovery after an illness or surgery
