RDW-CV Test: Normal Range, High & Low Levels

What is RDW-CV? Understanding Red Cell Distribution Width

When I review a complete blood count (CBC) with my patients, I often point out a lesser-known but remarkably useful parameter: the RDW-CV, or Red Cell Distribution Width–Coefficient of Variation. This simple number tells me how uniform the size of your red blood cells is. Think of it as a measure of diversity — a low RDW means your red cells are all roughly the same size (like a bag of identical marbles), whereas a high RDW indicates that there is a wide mix of smaller and larger cells (like a handful of pebbles and boulders).

The RDW-CV is calculated from the red blood cell histogram and is expressed as a percentage. It is a standard part of the CBC and is especially valuable when evaluating anaemia. The officially recognised LOINC code for RDW is 788-0, which covers both RDW-CV and RDW-SD variants; most laboratories report the CV version.

Normal RDW-CV Reference Ranges by Age and Gender

Normal values vary slightly between laboratories, but the following ranges are typical for a healthy adult population. Note that some laboratories use RDW-SD (standard deviation) instead — always check the unit on your report.

Note: Ranges may differ by laboratory. Always interpret your results with the reference range provided on your report.

When is RDW-CV Ordered?

The RDW-CV is almost always part of a full blood count (FBC). I typically look at it closely when a patient presents with fatigue, pallor, shortness of breath, or other symptoms suggestive of anaemia. It is also helpful in monitoring certain chronic conditions like chronic kidney disease, inflammatory bowel disease, or after treatment with iron, vitamin B12, or folate.

RDW-CV in Anaemia Classification

One of the most powerful uses of RDW-CV is to help distinguish between different types of anaemia. For example:

• Iron deficiency anaemia: Typically shows an elevated RDW-CV (>14.5%) because the bone marrow produces a mix of small (microcytic) and normal-sized cells as iron stores dwindle.
• Thalassaemia trait: RDW-CV is usually normal or only mildly elevated, despite the presence of microcytosis. This helps differentiate it from iron deficiency.
• Vitamin B12 or folate deficiency: RDW-CV is often significantly elevated due to the presence of large (macrocytic) cells alongside smaller cells.
• Aplastic anaemia or anaemia of chronic disease: RDW-CV is often normal because cell size remains relatively uniform.

What Does a High RDW-CV Mean?

In my clinical practice, a high RDW-CV (above 14.5% in adults) always prompts me to think about an underlying condition that is causing red blood cell production to become erratic. The most common causes include:

• Iron deficiency (the most frequent cause worldwide)
• Vitamin B12 or folate deficiency
• Recent blood loss or haemolysis
• Bone marrow disorders (e.g., myelodysplastic syndrome)
• Chronic liver disease or alcoholism
• After chemotherapy or transfusion

Is High RDW-CV Dangerous?

A high RDW-CV itself is not a disease but a marker. The underlying condition, however, may be serious. For example, a very high RDW (often >17-18%) has been linked in studies to increased cardiovascular risk and poorer outcomes in hospitalised patients. But remember: correlation is not causation. The real danger depends on what is driving the elevation. I always tell my patients, “The RDW is like a smoke alarm — it tells us something is happening, but we need to find the fire.”

What Does a Low RDW-CV Mean?

Low RDW-CV values (below 11.5%) are much less common and usually indicate that red cells are very uniform in size. This can be seen in:

• Chronic disease (e.g., anaemia of chronic disease)
• Thalassaemia trait (especially when the RDW is normal while MCV is low)
• Early-stage iron deficiency before size variation appears
• Healthy individuals with no haematologic issues — it’s not necessarily abnormal.

RDW-CV During Pregnancy

Pregnancy naturally causes a dilutional anaemia and increased erythropoiesis. The RDW-CV often rises slightly, especially in the second and third trimesters, due to the demands of iron and folate. A mild elevation (up to 15.5%) is usually normal, but values above 16% may indicate iron deficiency that warrants supplementation. I always advise pregnant women to have their iron stores checked if RDW starts climbing.

How the RDW-CV Test is Performed

The test requires a simple blood draw from a vein in your arm. No special preparation (fasting) is needed. The sample is sent to a laboratory where an automated analyser measures the volume of thousands of red cells, plots a histogram, and calculates the coefficient of variation. Results are typically available within a few hours.

Factors That Can Affect RDW-CV

A few things can temporarily alter your RDW-CV:

• Recent blood transfusion (introduces cells of different sizes)
• Haemolysis from a difficult blood draw
• Certain medications (e.g., chemotherapy drugs, antiretroviral therapy)
• Cold agglutinins (antibodies that cause red cells to clump)

If your result is borderline or unexpected, your doctor may repeat the test or order additional iron studies, B12, folate, and a peripheral blood smear.

A Final Word from the Clinic

I’ve seen many patients worry unnecessarily about an isolated high RDW. My message is always the same: the RDW is never interpreted in isolation. It is a puzzle piece that fits alongside haemoglobin, MCV, MCH, and your clinical history. Together, they paint a picture of your bone marrow’s health. If you have any concerns about your RDW-CV result, please discuss it with your healthcare provider — we are here to help you understand what it truly means.