RDW

Red cell distribution width (RDW) is a measure derived from the complete blood count that describes the variation in size of circulating red blood cells. RDW is reported as RDW-CV, the coefficient of variation of red blood cell volumes, and RDW-SD, the standard deviation of red blood cell volumes measured in femtoliters. Normal ranges vary by laboratory method, but typical RDW-CV values are about 11.5% to 14.5%, and RDW-SD around 39 to 46 fL.

RDW increases when there is pronounced anisocytosis, a wide range of red blood cell sizes. Causes include iron deficiency anemia, mixed deficiencies (iron together with B12 or folate), recent or ongoing blood loss, hemolytic anemia, inflammatory or liver diseases, and recovery after acute illness or transfusion. RDW is often interpreted with mean corpuscular volume (MCV): iron deficiency anemia commonly shows increased RDW with low MCV, whereas thalassemia trait often has a normal RDW with low MCV. In megaloblastic anemia or other deficiencies, RDW is frequently elevated.

RDW has limited diagnostic specificity but can aid differential diagnosis and may have prognostic value. Higher RDW has been associated with worse outcomes in conditions such as heart failure, stroke, and sepsis, and with all-cause mortality in some populations, though it is not a stand-alone test.

Limitations include laboratory variability and overlap among conditions. RDW should be interpreted as part of the full CBC and in the clinical context with other laboratory data.