Red Cell Distribution WidthRDW
Coefficient of variation of red cell size; an early and prognostic marker.
What it measures
RDW = SD of RBC volume / MCV × 100. Rises when erythropoiesis is heterogeneous (mixed populations of young and old, large and small cells).
Why we order it
Disambiguate causes of microcytosis (high in IDA, normal in thalassemia trait) and detect early or mixed nutritional deficiencies before MCV shifts.
Interpretation by lens
An isolated rise in RDW with normal MCV often precedes overt anemia by months — use it as an early warning.
Functional cutoff is ~13.0%; values above this in an otherwise 'normal' CBC warrant ferritin and B12.
RDW independently predicts all-cause mortality in older adults, ICU patients, and HF cohorts — likely as an integrated marker of inflammation, oxidative stress, and nutritional state rather than a specific disease.
Causes of abnormal values
- ↑Iron deficiency (often the earliest CBC change)
- ↑B12 / folate deficiency
- ↑Mixed deficiency
- ↑Recent transfusion
- ↑Hemolysis with reticulocytosis
- ↑Myelodysplasia
- ↓Generally not clinically meaningful; very low RDW has no established disease association.
Pre-analytic & interpretation traps
- !Recent transfusion artificially raises RDW for weeks. Note transfusion history before interpreting.