RDW

What is RDW?

RDW (Red cell Distribution Width) is a measure of the variation in cell size between individual red blood cells, expressed as a percentage (%). It is calculated as the standard deviation of the red cell volume divided by the mean cell volume (MCV), multiplied by 100. Reference values typically fall between 11.5 and 14.5%. An elevated RDW (anisocytosis) means red blood cells vary widely in size — both small and large cells are present in the same sample. RDW rises when the bone marrow produces cells under changing or stressed conditions. In early iron deficiency, the marrow is already making microcytic cells while normal-sized cells still circulate — two populations that increase variability before MCV falls overall. The same applies during recovery after blood loss, after iron or B12 supplementation, or with haemolytic anaemia: reticulocytes are larger than mature erythrocytes and increase the spread. RDW is therefore an early-change marker that can shift before MCV or haemoglobin deviate significantly. In thalassaemia trait, the bone marrow consistently produces small, uniform cells — MCV is low but RDW typically remains normal, because there are not two mixed populations. This is the classic distinction from iron deficiency, which does produce variable cell sizes (elevated RDW). In anaemia of chronic disease, RDW more often remains normal while MCV is mildly low-normal.

Why is RDW relevant?

RDW is clinically valuable because it detects early changes that MCV and haemoglobin have not yet captured, and because it helps distinguish anaemia patterns from one another. The most widely used diagnostic tool is the combination of MCV category and RDW status, also known as the RDW-MCV grid: low MCV + high RDW classically fits iron-deficiency anaemia; low MCV + normal RDW points more toward thalassaemia trait; high MCV + high RDW fits B12 or folate deficiency; high MCV + normal RDW fits liver disease, alcohol use, or medication-related macrocytosis; normal MCV + high RDW fits early iron or B12 deficiency, or a mixed deficiency that neutralises MCV. Elevated RDW is also associated with worse outcomes in heart failure, chronic kidney disease, and sepsis — not as a diagnostic tool but as a reflection of stress on erythropoiesis. This is more prognostic than actionable, but illustrates that RDW communicates more than anaemia alone. For people starting iron supplementation or B12 treatment, RDW is a useful early response indicator: it transiently rises as new cells of different sizes enter circulation, then normalises as production stabilises. A temporarily rising RDW in someone receiving B12 is therefore good news, not bad.

RDW high or low — what it means

RDW has its greatest diagnostic value in combination with MCV, haemoglobin, MCH, and MCHC. Use the MCV-RDW combination as the starting point for differentiating anaemia types, and follow up with ferritin and transferrin saturation for microcytosis, or with B12 and folate for macrocytosis. With normal MCV but elevated RDW and anaemia: think early iron deficiency or a mixed deficiency and test both. An isolated mildly elevated RDW (14.5–16%) with an otherwise normal blood count and no symptoms is not uncommon and calls for repeat testing and context, not immediate action. Chronic conditions, recent blood loss, or a recovery period after infection can transiently elevate it. Conversely, persistently elevated RDW in someone with anaemia or fitting symptoms warrants serious attention and follow-up testing. RDW is a stable, well-reproducible measurement and is little affected by pre-analytical variables. Blood transfusions can transiently elevate it by mixing donor and recipient cells of different sizes — a context to factor in with recent transfusions.

Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.