What do RDW (Red Cell Distribution Width)-cv (coefficient of variation) and RDW-SD (standard deviation) values indicate?

Understanding RDW-CV and RDW-SD

RDW-CV (coefficient of variation) and RDW-SD (standard deviation) both measure the variation in red blood cell size, but RDW-CV is the standardized percentage calculation used clinically, while RDW-SD provides the absolute variation in femtoliters—with RDW-CV being the primary clinical tool for diagnosing anemia causes and assessing disease severity. 1

Technical Definitions and Calculation

RDW-CV Calculation:

• RDW-CV is calculated by dividing the standard deviation of red blood cell volume by the MCV and multiplying by 100 to express as a percentage 1
• Formula: RDW (%) = {SD of red blood cell volume (fL)/MCV (fL)} x 100 1
• Normal range: ≤14.0%, corresponding to the 95th percentile in reference populations 2
• Typical normal range: 11.0-14.0% 2

RDW-SD:

• Represents the absolute standard deviation of red blood cell volumes measured in femtoliters 1
• Provides direct measurement of variation without normalization to MCV 1
• Less commonly used in clinical practice than RDW-CV 2

What These Values Tell You Clinically

Normal RDW (≤14.0%):

• Indicates homogeneous red cell population with uniform size (absence of anisocytosis) 2
• When combined with low MCV, suggests thalassemia minor rather than iron deficiency anemia 1, 2
• When combined with normal MCV and hemoglobin, suggests absence of early iron deficiency or vitamin deficiencies 2

Elevated RDW (>14.0%):

• When combined with low MCV, strongly indicates iron-deficiency anemia 1, 2
• With normal MCV, may indicate early iron deficiency, vitamin B12/folate deficiency, or hemolysis 2
• Reflects heterogeneous red cell population with significant size variation 3

Diagnostic Algorithm for Microcytic Anemia

Step 1: Assess MCV and RDW together 1, 2

• Low MCV + High RDW (>14.0%) = Iron deficiency anemia 1, 2
• Low MCV + Normal RDW (≤14.0%) = Thalassemia minor 1, 2

Step 2: Confirm with additional testing 2

• For suspected iron deficiency: serum ferritin (<30 μg/L without inflammation, <100 μg/L with inflammation) and transferrin saturation 2
• For suspected thalassemia: hemoglobin electrophoresis 4

Clinical Pitfalls and Caveats

Important Limitations:

• RDW values vary between different laboratory instruments, so interpret according to specific laboratory reference ranges 2
• In vitamin B12 deficiency (pernicious anemia), 31% of patients have normal RDW despite active disease 5
• RDW elevation is not necessarily the earliest indicator of vitamin B12 deficiency, unlike in iron deficiency 5
• Some patients with early vitamin B12 deficiency have normal RDW and normal MCV simultaneously 5

When Normal RDW Doesn't Rule Out Disease:

• Always interpret RDW alongside hemoglobin, MCV, and other red cell indices 2
• If clinical suspicion for iron deficiency exists despite normal RDW, proceed with serum ferritin and transferrin saturation testing 2
• A wide size range of red cells (high RDW) can help identify iron deficiency when microcytosis and macrocytosis coexist, resulting in falsely normal MCV 2

Beyond Anemia Diagnosis

Broader Clinical Applications:

• Elevated RDW reflects impaired erythropoiesis, shortened red cell lifespan, oxidative stress, and chronic inflammation 6, 3
• Associated with increased mortality risk in general population and multiple disease states including cardiovascular disease, cancer, and chronic inflammatory conditions 3
• In acute COVID-19 and inflammatory conditions, elevated RDW reflects erythrocyte membrane injury and reduced deformability 2

Minimum Workup for Elevated RDW:

• Complete blood count with MCV, reticulocyte count, serum ferritin, transferrin saturation, and CRP 2
• Peripheral blood smear to assess red cell morphology 4