RDW Blood Test: What Red Cell Distribution Width Tells You

What Is RDW?

Red cell distribution width (RDW) is a statistical measure of the variability in size (volume) of your red blood cells. It is calculated as the coefficient of variation of red cell volume — expressed as a percentage. A higher RDW means your red cells vary more in size; a lower RDW means they are more uniform.

Healthy red blood cells are produced in the bone marrow and should all be roughly the same size (about 6–8 micrometres in diameter). When the bone marrow is producing red cells normally and has adequate nutrients, the cells emerge uniform. When something disrupts production — nutrient deficiency, chronic disease, or abnormal bone marrow activity — cells of varying sizes are released, widening the distribution and raising RDW.

RDW Reference Ranges

RDW-CV (%)	Interpretation
11.5–14.5%	Normal — uniform red cell size
14.5–16.0%	Mildly elevated — early nutritional deficiency or mixed anaemia
16.0–18.0%	Moderately elevated — investigate iron, B12, folate; thalassaemia possible
Above 18.0%	High — significant red cell size variation; haemolysis, severe deficiency, or bone marrow issue
Below 11.5%	Low (uncommon) — highly uniform cells; seen in microcytic anaemia of chronic disease

RDW and MCV: The Anaemia Classification System

RDW is most useful when interpreted alongside MCV (mean corpuscular volume — the average red cell size). Together, these two measurements can narrow down the cause of anaemia before any other testing is done:

MCV	RDW	Most Likely Cause
Low (microcytic)	High	Iron deficiency anaemia (classic pattern)
Low (microcytic)	Normal	Thalassaemia trait or anaemia of chronic disease
Normal (normocytic)	High	Early iron or B12/folate deficiency; mixed deficiency
Normal (normocytic)	Normal	Anaemia of chronic disease; acute blood loss
High (macrocytic)	High	Vitamin B12 or folate deficiency (classic pattern)
High (macrocytic)	Normal	Liver disease, alcohol, hypothyroidism, medications

The combination of low MCV + high RDW is the textbook pattern for iron deficiency anaemia. The combination of high MCV + high RDW is the textbook pattern for B12 or folate deficiency. Normal MCV + high RDW is particularly useful — it can signal early deficiency before anaemia has fully developed, providing an earlier diagnostic window.

What Causes High RDW?

The most common causes of elevated RDW are nutritional deficiencies that disrupt normal red cell production. Iron deficiency is the single most common cause worldwide, followed by B12 and folate deficiency. When two deficiencies coexist (e.g., iron and B12 simultaneously), small and large cells are produced together, creating a very wide distribution — the MCV may paradoxically appear normal because the small and large cells average out.

RDW as a General Health and Inflammation Marker

Beyond anaemia, high RDW has emerged as a marker of systemic stress and chronic inflammation. Multiple large studies — including analyses from the NHANES dataset and European heart disease registries — have found that elevated RDW independently predicts cardiovascular mortality, all-cause mortality, hospitalisation, and worse outcomes across numerous chronic diseases including heart failure, chronic kidney disease, COPD, and diabetes.

The likely mechanism is that chronic inflammation impairs the bone marrow's ability to produce uniform red cells. Inflammatory cytokines interfere with erythropoiesis (red cell production), causing premature release of immature cells of varying size. RDW therefore rises not only with nutritional deficiencies but with any sustained state of systemic inflammation — making it a surprisingly broad marker of physiological stress.

When to Investigate a High RDW

If your RDW is elevated on a CBC, the next steps depend on whether anaemia is also present and what the MCV shows. With anaemia and elevated RDW, the combination of ferritin, B12, and folate testing will identify the most common causes. Without anaemia, a mildly elevated RDW (14.5–16%) warrants checking ferritin (which falls before haemoglobin does) and B12, and monitoring with repeat CBC in 3–6 months.

A very high RDW (above 18%) that persists after correcting nutritional deficiencies warrants haematology referral to rule out myelodysplastic syndrome — a bone marrow disorder where abnormal cell production is the primary problem. This is particularly relevant in older adults with unexplained macrocytosis and cytopenias.