RDW-CV Blood Test: What Your Results Mean

If you’re looking for information about the RDW-CV blood test, start here. This guide explains what the test measures and how to understand your results.

RDW-CV shows how much red blood cell sizes differ within one blood sample^2,4. Clinicians read it with MCV and other complete blood count results^1,4.

RDW-CV can help with anemia checks, but it cannot name a cause alone^1,4. Your laboratory range and other blood results provide needed context^1,2.

Red cell distribution width measures differences in red blood cell size and volume^1,4. Doctors call this size variation anisocytosis.

Red blood cells contain hemoglobin, a protein that carries oxygen throughout the body¹. Healthy red blood cells are often close to the same size⁴.

RDW does not measure the width of one red blood cell. Instead, “distribution width” describes the spread of cell sizes on a graph^4,7.

This graph is called a histogram⁴. Similar cell sizes cluster together, while varied sizes spread farther apart⁴.

A high result shows more size variation than expected⁴. A low result shows that the cells are close in size⁴.

Our guide to anisocytosis explains this finding in more detail.

RDW-CV reports red blood cell size variation as a percentage. The lab gets RDW-CV from the MCV curve.

MCV means mean corpuscular volume. It shows the average volume, or average size, of red blood cells^1,2.

The letters “CV” refer to the coefficient of variation used for this result. This calculation links the spread of cell volumes to their average size.

MedlinePlus lists RDW-SD as another name used for RDW testing⁴. RDW-CV is the percentage form described in clinical references.

If your report says RDW-SD, ask how the laboratory reports that result. Use the range printed with the exact test on your report.

RDW is one of several red blood cell indices in a complete blood count^1,2. A complete blood count, or CBC, measures the numbers and types of blood cells¹.

The main red blood cell indices include:

The CBC also includes red blood cell count, hemoglobin, and hematocrit. Our CBC blood test guide explains the wider panel.

No single red blood cell index can confirm a diagnosis^1,4. Clinicians consider the full group of results when checking for anemia¹.

A commonly cited adult RDW-CV normal range is about 11.5% to 15%. However, the correct range can differ between laboratories^2,4.

Always use the range printed next to your result. Local laboratory ranges should guide CBC interpretation.

A normal RDW-CV means cell sizes have an expected amount of variation⁴. The cells appear close together on the laboratory graph⁴.

However, normal RDW-CV does not prove the average cell size is normal⁴. Red blood cells may still be larger or smaller than expected⁴.

A normal result also does not rule out anemia or another condition^4,7. MCV and other CBC results may show changes that RDW-CV does not detect^4,7.

A high RDW-CV means red blood cell sizes vary more than expected^2,4. The cell sizes appear more spread out on the histogram⁴.

A high result may be a sign of anemia or a related condition⁷. However, RDW results do not always identify which condition is responsible for the variation^1,4.

Doctors may compare high RDW-CV with these CBC findings:

The combined findings offer more information than RDW-CV alone^1,4. Symptoms and medical history also help clinicians understand blood test results¹.

A low RDW-CV means the red blood cells are very similar in size⁴. Their measurements sit close together on the histogram⁴.

Low RDW is not a sign of anemia^4,7. It is not viewed as a sign of an underlying condition⁷.

Other CBC findings may still fall outside their listed ranges. Review the whole report rather than focusing only on RDW-CV.

MCV and RDW-CV answer different questions. MCV shows average cell size, while RDW-CV shows size variation^1,2.

MCV helps group anemia by average red blood cell size. These groups are called microcytic, normocytic, and macrocytic anemia.

Microcytic means the average cell size is low. Normocytic means it is within range, while macrocytic means it is high.

These patterns help organize a clinical review. They do not confirm one cause without other information^1,4.

Low MCV means red blood cells are smaller than expected on average¹. It may occur with iron deficiency anemia or thalassemia¹.

Thalassemia is a group of inherited blood disorders¹. These disorders can reduce healthy red blood cells and hemoglobin¹.

High RDW-CV adds a second finding. It shows that the individual cell sizes also differ more than expected^2,4.

Normal RDW-CV means the cells have an expected amount of size variation⁴. It does not identify why MCV is low.

Neither pattern confirms iron deficiency or thalassemia by itself^1,4. Clinicians review the remaining CBC findings before drawing conclusions⁶.

Normal MCV means the average cell volume falls within the laboratory range. However, an average may hide two groups of differently sized cells.

Small and large cells can produce an average MCV within range. A raised RDW-CV may show that the individual sizes remain widely spread^3,4.

This pattern shows why MCV should not be used alone. Clinicians may review other indices and examine a peripheral blood smear.

A peripheral smear lets the laboratory examine blood cell form under close review. It can show cell shapes and other factors affecting the results.

High MCV means red blood cells are larger than expected on average^1,2. It may occur with vitamin B deficiency or liver disease¹.

High RDW-CV means the cell sizes also vary more than expected⁴. Normal RDW-CV means the amount of variation remains within range⁴.

RDW-CV cannot separate vitamin deficiency, liver disease, or other explanations by itself^1,4. The rest of the CBC remains important⁵.

These steps can help you prepare for a clinical discussion:

These steps do not replace a clinician’s review. They can help you understand how the results fit together.

A clinician may order a CBC during a routine health check^1,4. Testing may also occur when the clinician suspects anemia^1,5.

Anemia occurs when the body lacks enough healthy red blood cells¹. Symptoms listed in the sources include:

These symptoms cannot establish anemia by themselves. Clinicians consider them with medical history and blood test results¹.

Someone without symptoms may still receive RDW-CV through a routine CBC^1,4. The reason for testing can affect how a clinician reads the result.

High RDW-CV has no single explanation because it only measures size variation^1,4. The result cannot identify a specific nutrient problem by itself.

Combined blood findings may help clinicians assess iron deficiency⁴. Low MCV may also occur with iron deficiency anemia¹.

A diet low in iron and other minerals can prompt RDW testing⁵. Blood loss from an injury or surgery can also be a reason for testing⁴.

High MCV may occur with anemia caused by vitamin B deficiency¹. However, RDW-CV alone cannot confirm that cause^1,4.

RDW results may help with thalassemia testing when combined with other findings⁴. A family history of sickle cell anemia may also prompt RDW testing⁴.

A family history of inherited blood disorders may lead to RDW testing⁴. Examples include thalassemia and sickle cell anemia⁴.

Family history provides context, but it does not explain an RDW-CV result alone. A clinician must review the complete set of findings^1,4.

Some chronic illnesses may be reasons for ordering an RDW test⁴. MedlinePlus lists Crohn’s disease, diabetes, and HIV as examples⁴.

A long-term infection may also lead a clinician to order testing⁴. These details help explain why the test was requested.

They do not prove why RDW-CV is high. The blood result still needs clinical review⁵.

Other blood findings can help clarify an abnormal RDW-CV result^1,4. A clinician may also order more tests to find the cause⁷.

A standard CBC offers several related measurements:

Clinicians may also watch CBC values over time⁵. Past and current results can provide useful context⁵.

RDW-CV testing uses a blood sample from a vein in the arm^1,4. A health professional collects a small amount in a tube or vial¹.

The blood draw often takes less than five minutes^1,4. The needle may cause a brief sting when it enters or leaves¹.

A CBC does not need special preparation^1,4. Other tests ordered at the same time may require fasting⁵.

Blood tests carry little risk^1,4. Mild pain or bruising can occur where the needle entered^1,4.

Some people feel lightheaded after a blood draw⁷. A clinician may suggest waiting until that feeling passes before leaving⁷.

Treatment may be needed if anemia or another medical condition is found⁴. RDW-CV alone does not give enough information for a treatment choice^1,4.

Depending on the condition, a clinician may suggest supplements, medicines, or diet changes⁴. The right choice depends on the full test results^1,4.

High RDW-CV does not prove iron deficiency^1,4. Other forms of anemia can also cause abnormal red blood cell findings^1,4.

Talk with your clinician before taking supplements or changing your diet⁴. A clinician can decide whether iron matches the condition found during testing⁴.

If treatment is prescribed, ask how follow-up will work. You can also ask when the CBC may need another review.

The following prompts can support a clear discussion:

Discuss an out-of-range RDW-CV result with the clinician who ordered the test. RDW needs other blood findings and medical context for interpretation^1,4.

Contact your clinician if fatigue or pale skin gets worse⁷. Also contact your clinician if an irregular heartbeat gets worse⁷.

Speak with a healthcare professional when your RDW-CV result is unclear⁷. Ask what the result means beside your other CBC values.

A laboratory flag does not establish a diagnosis^1,4. A result within range also does not rule out every type of anemia^4,7.

Understanding your RDW-CV blood test results begins with one main point: RDW-CV measures differences in red blood cell size^2,4.

A high result means cell sizes vary more than expected⁴. A low result reflects similar sizes and is not a sign of anemia^4,7.

Use the RDW-CV normal range shown on your report^2,4. Review RDW-CV with MCV, hemoglobin, other CBC findings, and your medical history^1,2.