Complete Blood Count (CBC)Reported in %Red cell distribution width (RDW)
The percent variation in red blood cell size. A higher RDW means the cells are less uniform.
What it measures
RDW is the coefficient of variation of red cell volume, reported as a percent. Healthy marrow tends to release cells of a similar size, so the percent stays in a narrow band. When the factory changes its output, the percent rises.
Why it is on the panel
It is one of the more useful quiet numbers on a CBC. People with a normal hemoglobin and a rising RDW are sometimes at the start of an iron or B12 problem that the average size has not revealed yet.
How to read it
The classic teaching pair is MCV plus RDW. Low MCV and high RDW leans toward iron deficiency. Low MCV and normal RDW leans toward thalassemia trait. High MCV and high RDW fits B12 or folate deficiency more often than a uniformly large-cell condition.
When it runs higher
Iron deficiency, B12 or folate deficiency, recent bleeding, and recovery from anemia all widen RDW. Inflammatory illness can nudge it up as well.
When it runs lower
A low RDW is rarely treated as a finding on its own. Very uniform cells are usually just uniform cells.
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
RDW 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.