Red Cell Distribution Width (RDW) is a parameter that reflects the variability in RBC size (anisocytosis).

RDW-SD specifically measures the standard deviation of RBC volume in femtoliters (fL).

Clinical uses include:

Differentiating iron deficiency anemia from thalassemia.

Supporting diagnosis of megaloblastic anemia (e.g., vitamin B12 or folate deficiency).

Evaluating mixed anemia conditions.

Monitoring treatment response in anemia patients.

The test is performed as part of a Complete Blood Count (CBC) using automated hematology analyzers.

Analyte: Variation in red blood cell size (RBC volume distribution).

Normal Range: ~39–46 fL (varies slightly by lab).

Abnormal Findings:

High RDW-SD: Suggests anisocytosis, commonly seen in iron deficiency anemia, megaloblastic anemia, or mixed anemia.

Normal RDW-SD with anemia: May suggest thalassemia trait or chronic disease anemia.

Clinical Use:

Provides quantitative measure of RBC size variability.

Helps distinguish between different types of anemia.

Guides further diagnostic evaluation and treatment planning.