Red cell distribution width (SD)

fL · Full blood count & differentials

RDW-SD reports red-cell size variation in femtolitres — the absolute-width counterpart to RDW-CV (%).

Labs may report RDW-CV, RDW-SD, or both. Rising RDW with falling MCV supports iron-deficiency patterns; isolated RDW shifts need context from the full FBC.

How to read it

Use the matching catalog marker (rdw vs rdw_sd) for the unit your report prints. Trends with haemoglobin and ferritin matter more than a single draw.