What does it measure?
RDW describes the spread of red-cell sizes in your sample. MCV gives an average, while RDW helps show whether most cells are similar or whether small and large cells are mixed together. It is usually reported automatically as part of the CBC. [1]
Before the test
- Sample
- Blood, usually collected with a complete blood count.
- Fasting
- The CBC alone usually needs no fasting. Other tests at the appointment may have different instructions.
What can the result mean?
A higher RDW means more variation in size. That can happen with iron or vitamin deficiencies, mixed causes of anemia, or changing red-cell populations during recovery. It does not identify a specific deficiency. A low RDW is usually less useful clinically and is not a target to raise. [1]
When is it useful, and how is it read?
In an anemia assessment, low MCV with high RDW can support an iron-deficiency pattern, while other combinations point elsewhere. These patterns overlap, so ferritin, symptoms, and sometimes a blood film matter more than RDW alone. [3] [4]
What can affect it?
The meaning depends on hemoglobin and MCV, as well as recent transfusions or treatment. Laboratories may report RDW-CV as a percentage or RDW-SD as a volume, so check the label before comparing different reports. [1]
Myth: «High RDW means all the cells are large»
RDW describes how much cell sizes vary, not whether every cell is large. A high result can include small cells, large cells, or a mixture. MCV supplies the average size that RDW cannot. [3] [4]
Questions you might have
Does a high RDW mean the cells are all large?
No. It means their sizes vary more. MCV tells you the average size. [1]
Can RDW diagnose iron deficiency alone?
No. A size pattern needs hemoglobin, MCV and, when relevant, iron results to explain it. [1]
What comes next?
If RDW is flagged, first ask whether you actually have anemia and what the other red-cell measurements show. Your clinician may use ferritin, B12, folate, or a blood smear when the pattern calls for them. Avoid turning a size-variation result into a diagnosis of general inflammation or aging. [1]
What does the combination of my RDW, MCV, and hemoglobin tell us?
The reference interval and units on your report are part of understanding your result. Understand units, reference intervals, and treatment targets.
Sources & references
Medical references & original research
Books we consulted
References that informed our explanations of the test and its context.
- Pagana KD, Pagana TJ, Pagana TN. Mosby’s Diagnostic and Laboratory Test Reference. 14th ed. Elsevier; 2019.
- Provan D, editor. Oxford Handbook of Clinical and Laboratory Investigation. 4th ed. Oxford University Press; 2018.
Practical-section sources accessed: .
For general health education. Interpreting your result and deciding on tests or treatment requires your health context and a qualified clinician.
