Complete Blood Count (CBC)Reported in fLRed cell distribution width, standard deviation (RDW-SD)
How spread out red cell sizes are, reported as a width in femtoliters. It is the companion to RDW percent.
What it measures
RDW-SD measures the width of the red-cell size curve directly, in femtoliters. A narrow curve means the cells are a similar size. A wide curve means a mix of small and large cells is circulating at the same time.
Why it is on the panel
Early nutrient deficiency often shows up as uneven cells before the average size (MCV) has fully shifted. RDW-SD catches that spread.
How to read it
Read it with RDW and MCV. Both RDW numbers tell the same story in different units. If one is high and the other is typical, the report interval and the units are the first thing to check.
When it runs higher
A wider distribution is common when the marrow is releasing cells of a new size: early iron deficiency, mixed deficiency, recent treatment of anemia, or recovery from bleeding. Transfusion can widen it too, because donor cells mix with your own.
When it runs lower
A very narrow distribution is less often meaningful. Uniformly small cells, as in some thalassemia traits, can keep RDW fairly tight even when MCV is low.
How to use this page
This page explains what the measurement is for. It is educational. Your Healiam report includes the reference interval for your age and sex, and a clinician reads that result with your history, medicines, and the rest of the panel. Talk with a clinician before changing diet, supplements, or treatment because of a number on a report.
RDW-SD
RDW-SD is part of the full panel.
No referral or insurance required.
One preventive draw covers this measurement and the rest of the Healiam panel. CLIA-certified results come back in days.