Understanding Red Cell Distribution Width Standard Deviation (RDW-SD)
RDW-SD is a measurement of red blood cell size variation (anisocytosis) expressed as the standard deviation of red blood cell volume in femtoliters (fL), providing a direct assessment of the degree of red cell size heterogeneity. 1
Definition and Calculation
- RDW-SD is calculated as the standard deviation of red blood cell volume, while RDW-CV (coefficient of variation) is calculated by dividing the standard deviation of red blood cell volume by the mean corpuscular volume (MCV) and multiplying by 100 to express as a percentage 1
- RDW-SD is a direct measurement of the width of the RBC size distribution curve in absolute values (femtoliters), making it less dependent on MCV than RDW-CV 1
- Normal RDW-SD values vary between laboratories but typically range between 39-46 fL 1
Clinical Significance
- RDW-SD reflects the degree of heterogeneity in erythrocyte volume and is traditionally used in laboratory hematology for differential diagnosis of anemias 2
- An increased RDW-SD indicates greater variation in red blood cell sizes, which can be seen in various conditions including iron deficiency anemia, vitamin B12 or folate deficiency, and hemolytic anemias 1
- When evaluating microcytic anemia, RDW-SD helps distinguish between causes, with iron deficiency anemia typically presenting with elevated RDW-SD and low MCV 1, 3
- Thalassemia minor typically presents with low MCV but normal or only mildly elevated RDW-SD, making this parameter useful in differential diagnosis 1
Interpretation in Clinical Practice
- RDW-SD should always be interpreted alongside other hematologic parameters including hemoglobin, MCV, and other red cell indices 1
- In iron deficiency anemia, RDW-SD shows an inverse relationship with hemoglobin levels and transferrin saturation - higher RDW-SD values correlate with more severe iron deficiency 3
- A RDW-SD above normal range strongly suggests the presence of iron deficiency anemia when differentiating from thalassemia trait 3
- Even with normal RDW-SD, if anemia is present, further investigation should include red blood cell indices, reticulocyte count, differential blood cell count, serum ferritin, transferrin saturation, and CRP concentration 1
Expanded Clinical Applications
- Beyond hematological disorders, elevated RDW-SD has been associated with cardiovascular diseases, venous thromboembolism, cancer, diabetes, respiratory diseases, and liver and kidney failure 2, 4
- RDW-SD has emerged as a strong and independent risk factor for mortality in the general population 2, 5
- In polycythemia vera, RDW-SD tends to be higher than normal and may reflect pathologic status, increasing during treatment transitions 6
- RDW-SD can transiently increase following myelosuppressive therapy in polycythemia vera patients, corresponding to the transition period from microcytes to normal blood cells 6
Technical Considerations
- RDW-SD values can vary between different laboratory instruments, so values should be interpreted according to the specific laboratory's reference range 1
- When counting at least 2,500 gated lymphocytes in flow cytometry, results are within 2% standard deviation of the "true" value with 95% confidence (binomial sampling) 7
- Laboratory variability should be determined by preparing and analyzing at least six replicates when validating methodology 7
Common Pitfalls
- Failing to interpret RDW-SD in conjunction with other red cell parameters can lead to misdiagnosis 1
- Not considering that RDW-SD values vary between different laboratory instruments and reference ranges 1
- Overlooking that normal RDW-SD with abnormal MCV still warrants investigation for potential underlying conditions 1
- Ignoring the prognostic value of RDW-SD beyond its diagnostic utility in anemia classification 2, 5