What this marker measures
RDW is particularly useful when combined with MCV. For example, iron deficiency often raises RDW before MCV changes, making it an early clue to nutritional deficiency.
High or low: what it can mean
High RDW
A high RDW (anisocytosis) indicates a wide variation in red blood cell size. This is commonly seen in iron deficiency, B12 or folate deficiency, and mixed anemias.
Read full explanation →
Low RDW
A low RDW is uncommon and generally not clinically significant on its own. It indicates very uniform red blood cell sizes.
Read full explanation →
Frequently asked questions
What is RDW?
+
RDW measures the variation in size among red blood cells. A high RDW means cells vary considerably in size; a normal RDW means they are fairly uniform.
What does RDW measure?
+
RDW is particularly useful when combined with MCV. For example, iron deficiency often raises RDW before MCV changes, making it an early clue to nutritional deficiency.
What does high RDW mean?
+
A high RDW (anisocytosis) indicates a wide variation in red blood cell size. This is commonly seen in iron deficiency, B12 or folate deficiency, and mixed anemias.
What does low RDW mean?
+
A low RDW is uncommon and generally not clinically significant on its own. It indicates very uniform red blood cell sizes.
Related biomarkers
Editorial note
LabTestly explains lab markers using plain-English educational content. Reference ranges may vary by laboratory and should be interpreted with a clinician.
Educational content · Editorially reviewed for clarity · Last updated
Common sources used for lab-test education
Not medical advice
This page is for general understanding only and does not replace medical advice from your clinician. Reference ranges can vary by laboratory, age, sex, pregnancy status, medications, and clinical context.