RDWs

Red cell distribution width (RDW) is a standard parameter reported in complete blood counts that describes the variation in red blood cell (RBC) size, or anisocytosis. RDW is reported as two indices: RDW-CV, the coefficient of variation of RBC volume, and RDW-SD, the standard deviation of RBC volume. Both are derived from automated measurements of the red cell size distribution by hematology analyzers.

Normal ranges vary by instrument and laboratory. In many settings, RDW-CV commonly falls roughly between 11.5% and 14.5%, while RDW-SD is often in the vicinity of 38 to 46 femtoliters, though specific references differ. Abnormal RDW values are frequently encountered in anemia workups and can provide additional information beyond mean corpuscular volume (MCV) alone.

In clinical use, RDW helps in the interpretation of anemia patterns. Iron deficiency anemia frequently shows an elevated RDW due to a broader mix of small and normal-sized cells, especially as deficiency progresses. Thalassemia trait often presents with a normal RDW. B12 or folate deficiency can raise RDW, typically with macrocytosis. RDW can also be increased in mixed deficiencies, inflammatory states, or after recent transfusions. It is not a diagnostic test on its own but complements MCV and other laboratory data.

RDW has also been studied as a prognostic marker. Higher RDW values have been associated with worse outcomes in cardiovascular disease, heart failure, stroke, and sepsis in observational studies. The exact mechanisms are not fully understood and RDW is considered a nonspecific marker of perturbations in erythropoiesis, inflammation, or nutritional status.

Limitations include dependence on analyzer systems, preanalytic factors, reticulocytosis, transfusion history, and sample handling. Interpretation should consider the full CBC context and laboratory reference ranges.