RDW (Red Cell Distribution Width) Biomarker Testing
RDW shows how varied your red blood cell sizes are and can help explain anemia patterns; order through Vitals Vault and test at Quest locations.
With Vitals Vault, you have access to a comprehensive range of biomarker tests.
RDW stands for red cell distribution width. It is a number on your complete blood count (CBC) that describes how much your red blood cells vary in size.
RDW is not a diagnosis by itself, but it can be a useful clue. When you look at RDW alongside hemoglobin, hematocrit, and MCV (mean corpuscular volume), you can often narrow down whether an anemia pattern fits iron deficiency, vitamin B12/folate deficiency, recent blood loss, or mixed causes.
If your RDW is flagged high or low, the most helpful next step is usually not to focus on RDW alone. Instead, you use it to decide which follow-up labs (like iron studies or B12) make sense and when to recheck.
Do I need a RDW test?
You usually get RDW as part of a CBC, not as a standalone test. You may want a CBC that includes RDW if you have fatigue, shortness of breath with exertion, frequent headaches, paleness, dizziness, or unexplained weakness—especially if you also have heavy menstrual bleeding, a restricted diet, digestive symptoms, or a history of anemia.
RDW is also commonly checked when a prior CBC showed low hemoglobin or an abnormal MCV. In that setting, RDW helps your clinician interpret whether your red blood cells are becoming more uneven in size, which can happen when your body is making new cells under stress or when there are multiple nutritional issues at the same time.
You might also recheck RDW after starting treatment for anemia (such as iron or B12) because RDW can change as new red blood cells enter circulation. Testing supports clinician-directed care and follow-up planning, but it cannot confirm a specific cause of anemia on its own.
RDW is measured on automated hematology analyzers in CLIA-certified laboratories; results should be interpreted with the rest of your CBC and clinical context, not used as a standalone diagnosis.
Lab testing
Ready to check RDW? Order a CBC through Vitals Vault and complete your draw at Quest.
Schedule online, results typically within about a week
Clear reporting and optional clinician context
HSA/FSA eligible where applicable
Get this test with Vitals Vault
If you want a clear, practical way to check RDW, you can order a CBC through Vitals Vault and complete your blood draw at a nearby Quest location. RDW is typically reported automatically with your CBC, so you get the surrounding context that makes the number meaningful.
Once your results are in, PocketMD can help you make sense of what “high,” “normal,” or “low” RDW might mean alongside MCV, hemoglobin, and other CBC markers. That makes it easier to decide whether you should discuss iron studies, vitamin B12/folate testing, inflammation markers, or a repeat CBC with your clinician.
If you are tracking a known issue, Vitals Vault also makes it straightforward to retest on a timeline that matches how red blood cells change (often weeks, not days).
- Order online, then test at Quest locations
- Results you can trend over time in one place
- PocketMD guidance to prepare for a clinician visit
Key benefits of RDW testing
- Adds context to a CBC by showing how uniform (or varied) your red blood cell sizes are.
- Helps differentiate common anemia patterns when interpreted with MCV, hemoglobin, and RBC count.
- Can suggest mixed causes of anemia (for example, iron deficiency plus B12/folate deficiency) when other indices look borderline.
- Supports follow-up decisions about iron studies, B12/folate testing, and evaluation for blood loss when anemia is present.
- May change during recovery from anemia treatment, offering a clue that new red blood cells are entering circulation.
- Provides an additional data point when symptoms do not match a “normal” hemoglobin result.
- Makes it easier to have a focused conversation with your clinician by pairing a simple number with targeted next tests.
What is RDW?
RDW (red cell distribution width) describes the variation in the size of your red blood cells. If most of your red blood cells are about the same size, RDW is lower. If there is a wider mix of small and large cells, RDW is higher.
RDW is reported as part of a CBC and is calculated from the distribution of red blood cell volumes measured by an analyzer. Many labs report RDW-CV (a percentage) and some also report RDW-SD (a value in femtoliters). Your report may flag one or both depending on the lab.
RDW becomes most useful when you interpret it with other red blood cell indices. MCV tells you the average red blood cell size, while RDW tells you how spread out the sizes are around that average. Two people can have the same MCV but very different RDW values, which can point toward different underlying causes.
How RDW fits into anemia workups
Anemia is not one condition; it is a pattern where your blood has less oxygen-carrying capacity than expected, often reflected by low hemoglobin or hematocrit. RDW helps you understand whether your bone marrow is producing red blood cells that are consistently sized or whether there is a mix that suggests changing production, nutrient shortages, or recovery after blood loss.
RDW is a clue, not a verdict
A high RDW does not automatically mean you have iron deficiency, and a normal RDW does not rule it out. RDW is best used to guide what to check next—such as ferritin and iron saturation, vitamin B12 and folate, reticulocyte count, or evaluation for bleeding—based on your symptoms and the rest of your CBC.
What do my RDW results mean?
Low RDW
A low RDW generally means your red blood cells are very similar in size. In most cases, this is not considered a problem and is less clinically meaningful than a high RDW. If you have symptoms but RDW is low, your clinician will usually focus more on hemoglobin, MCV, and other CBC findings to look for explanations that do not rely on size variability.
In-range (typical) RDW
An in-range RDW suggests your red blood cells have a typical spread of sizes for the lab’s method. This can be reassuring, but it does not by itself rule out anemia or early nutrient deficiency. If hemoglobin is low, MCV is abnormal, or you have persistent symptoms, the rest of the CBC and targeted follow-up tests still matter.
High RDW
A high RDW means there is more variation in red blood cell size than expected. This often happens when your body is producing new red blood cells under changing conditions, such as iron deficiency, vitamin B12 or folate deficiency, recent blood loss, or recovery after treatment. High RDW is most informative when paired with MCV: high RDW with low MCV can fit iron deficiency, while high RDW with high MCV can fit B12/folate deficiency, alcohol effects, or certain medication patterns.
Factors that influence RDW
RDW can shift when your red blood cell production changes, because older cells and newly produced cells may differ in size. Recent bleeding, iron therapy, B12/folate replacement, and conditions that affect bone marrow output can all affect RDW over weeks. Inflammation and chronic illness can complicate interpretation by changing iron handling and red blood cell lifespan, so ferritin, transferrin saturation, and markers like CRP may be helpful companions. Lab methods and reference ranges vary, so it is best to compare your result to the range on your report and to your own prior results when available.
What’s included
Frequently Asked Questions
What is a normal RDW range?
Ranges vary by lab and by whether RDW is reported as RDW-CV (%) or RDW-SD (fL). Many labs flag RDW-CV as high when it is roughly above the mid–14% range, but you should use the reference interval printed on your report and compare to your prior results when possible.
What does high RDW mean if my hemoglobin is normal?
High RDW with normal hemoglobin can happen early in nutrient deficiency (especially iron, B12, or folate), during recovery after blood loss, or when there are mixed red blood cell populations. It is a reason to look at MCV and consider follow-up tests like ferritin, iron/TIBC, transferrin saturation, and B12/folate based on your symptoms and risk factors.
Can dehydration cause high RDW?
Dehydration more commonly affects concentration-based values like hemoglobin and hematocrit rather than changing the size distribution of red blood cells. RDW is less sensitive to hydration status than some other CBC markers, but interpretation should still consider the full CBC and your clinical situation.
Do I need to fast for an RDW test?
Fasting is not usually required for a CBC that includes RDW. If you are combining your CBC with other tests (like lipids or glucose), those may have fasting instructions, so follow the directions provided for your full order.
How is RDW different from MCV?
MCV is the average size of your red blood cells. RDW describes how much the sizes vary from one cell to another. Together, they help narrow down anemia patterns—for example, low MCV with high RDW often points toward iron deficiency, while high MCV with high RDW can suggest B12/folate deficiency or other causes of larger cells.
How often should RDW be rechecked?
That depends on why you are testing. If you are evaluating symptoms or a new abnormal CBC, a clinician may repeat a CBC in a few weeks to months, especially after starting treatment. Because red blood cells live about 120 days, meaningful changes often take weeks rather than days.
What follow-up tests are commonly ordered with abnormal RDW?
Common next steps include iron studies (ferritin, serum iron, TIBC, transferrin saturation), vitamin B12 and folate, reticulocyte count, and sometimes inflammation markers like CRP. If blood loss is suspected, evaluation may include stool testing or gynecologic assessment, depending on your situation.