Unit Converter
Red Blood Cell Distribution Width (RDW-SD)
(Absolute Measure of RBC Size Variation - Independent of MCV, Useful in Diagnosing Mixed & Early Anemias)
Synonyms
- RDW-SD
- Red cell distribution width (SD)
- RBC size distribution width
- Red cell size dispersion
- RBC histogram width
Units of Measurement
- fL (femtoliters)
- µm³ (cubic micrometers)
Conversion
1 fL=1 µm31\ \text{fL} = 1\ \text{µm}^31 fL=1 µm3
(These units are identical numerically, both representing cell volume.)
Description
RDW-SD is an absolute measure of the width of the RBC volume distribution curve (histogram).
It represents the range in RBC size (anisocytosis) in femtoliters.
Unlike RDW-CV (which is a relative percentage), RDW-SD:
- Is independent of MCV
- Represents true width of RBC size distribution
- More sensitive in detecting mixed anemia and early size variation before MCV changes
How measured (automated analyzers):
RDW-SD is taken at the 20% height of the RBC histogram and expressed as the width in fL.
Physiological Role
RDW-SD itself has no physiologic function; it is a measurement of RBC population heterogeneity, used to classify and diagnose anemias.
Clinical Significance
HIGH RDW-SD
Indicates increased anisocytosis (broad distribution of RBC sizes).
Major Causes
1. Iron Deficiency Anemia
- Early marker
- RDW-SD rises before MCV decreases
2. Vitamin B12 / Folate Deficiency
Macrocytes + microcytes → wide histogram → high RDW-SD.
3. Mixed Anemia
(iron deficiency + B12/folate deficiency)
RDW-SD often very high (>50 fL).
4. Hemoglobinopathies
- Thalassemia (may be high or normal)
- HbH disease
- Sickle cell disease
5. Recent Blood Transfusion
Presence of different-aged RBC populations.
6. Reticulocytosis
Reticulocytes are larger → widen distribution.
7. Liver Disease
Macrocytosis + cell variability.
8. Myelodysplastic Syndrome
Marked anisocytosis.
LOW / NORMAL RDW-SD
Normal RDW-SD usually indicates uniform RBC size.
Patterns
- Normal RDW-SD + Low MCV → thalassemia trait
- Normal RDW-SD + Normal MCV → anemia of chronic disease, acute bleeding
- Normal RDW-SD + High MCV → uniform macrocytosis (e.g., alcoholism)
Low RDW-SD has no clinical significance.
Reference Intervals
(Tietz 8E + WHO + Mayo + ARUP + Sysmex/Beckman Analyzer Standards)
RDW-SD
- 39 – 46 fL
(= 39 – 46 µm³, identical)
Interpretation
| RDW-SD (fL) | Interpretation |
| <38 fL | Uniform RBC population (clinically insignificant) |
| 39–46 fL | Normal |
| 47–55 fL | Mild anisocytosis |
| >55 fL | Marked anisocytosis (consider dual pathology) |
Diagnostic Uses
1. Early Detection of Iron Deficiency
RDW-SD rises before change in MCV.
2. Differentiating Iron Deficiency vs Thalassemia Trait
- High RDW-SD → iron deficiency
- Normal RDW-SD → thalassemia trait
3. Diagnosing Mixed Nutritional Anemia
Large range of RBC sizes → very high RDW-SD.
4. Monitoring Iron / B12 Therapy
Reticulocytosis widens RDW-SD initially, then normalizes.
5. Characterizing MDS & Bone Marrow Disorders
Markedly elevated RDW-SD.
6. Distinguishing Alcohol-Related Macrocytosis
Macrocytes (uniform) → RDW-SD normal or mild ↑.
Analytical Notes
- Derived from RBC histogram (20% height)
- Less influenced by MCV than RDW-CV
- Hemolysis, clotted samples, or cold agglutinins may distort histogram
- Always interpret with MCV, RBC count, and peripheral smear
Clinical Pearls
- RDW-SD is superior to RDW-CV when MCV is abnormal, because CV depends on MCV.
- Very high RDW-SD (>55 fL) → strongly suspect mixed anemia or MDS.
- RDW-SD + MCV patterns:
- Low MCV + High RDW-SD → iron deficiency
- Low MCV + Normal RDW-SD → thalassemia trait
- High MCV + High RDW-SD → B12/folate deficiency
- High MCV + Normal RDW-SD → uniform macrocytosis (e.g., alcoholism)
- Low MCV + High RDW-SD → iron deficiency
Interesting Fact
RDW-SD measures RBC variation in absolute units, making it the only CBC index that quantifies anisocytosis without influence from the mean RBC size (MCV).
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Hematology Indices
- WHO Hematology Standards
- Mayo Clinic Laboratories - RDW-SD
- ARUP Consult - CBC Interpretation
- Sysmex XN Series Manuals
- Beckman Coulter LH/UniCel Series Manuals
- MedlinePlus / NIH - RDW Test