What is the meaning of RDW (Red Cell Distribution Width)?

What is RDW (Red Cell Distribution Width)?

RDW (Red Cell Distribution Width) is a laboratory parameter that measures the variation in size of circulating red blood cells, calculated by dividing the standard deviation of red blood cell volume by MCV and multiplying by 100 to express as a percentage. 1

Normal Values and Interpretation

• Normal RDW is generally ≤14.0%, which corresponds to the 95th percentile in reference populations 1
• The typical reference range is 11.0-14.0% 1
• A normal RDW indicates a homogeneous red cell population without significant anisocytosis (variation in cell size) 1

Clinical Significance in Anemia Diagnosis

RDW is primarily used to differentiate between causes of anemia, particularly distinguishing iron deficiency from thalassemia:

• Iron deficiency anemia typically presents with elevated RDW (>14.0%) combined with low MCV, reflecting heterogeneous red cell populations 1, 2
• Thalassemia minor typically presents with low MCV but normal or only mildly elevated RDW (≤14.0%), indicating homogeneous microcytic cells 1, 2
• Anemia of chronic disease often shows normal or low RDW 2

Diagnostic Patterns

High RDW Clinical Scenarios:

• Microcytic anemia with high RDW strongly suggests iron deficiency anemia 1
• Normocytic anemia with high RDW may indicate early iron deficiency, vitamin B12/folate deficiency, or hemolysis 1
• High RDW can identify iron deficiency when microcytosis and macrocytosis coexist, resulting in a falsely normal MCV 1

Normal/Low RDW Clinical Scenarios:

• Low MCV with low RDW strongly suggests thalassemia minor rather than iron deficiency 2
• Normal MCV with low RDW may indicate early anemia of chronic disease 2
• Critical pitfall: Low RDW makes iron deficiency less likely, not more likely—never assume iron deficiency based solely on low hemoglobin with low RDW 2, 3

Essential Workup When RDW is Abnormal

The minimum laboratory evaluation should include: 1

• Complete blood count with MCV
• Reticulocyte count
• Serum ferritin (<30 μg/L suggests iron deficiency without inflammation; <100 μg/L with inflammation) 1, 2
• Transferrin saturation (<30% supports iron deficiency) 2
• C-reactive protein (CRP) to assess for inflammation 2
• Hemoglobin electrophoresis if thalassemia suspected based on ethnicity and low MCV with normal/low RDW 2

Beyond Hematology: Prognostic Value

RDW has emerged as a prognostic marker beyond anemia diagnosis:

• Elevated RDW is associated with increased morbidity and mortality in cardiovascular diseases including acute coronary syndrome, heart failure, atrial fibrillation, and stroke 4, 5, 6
• Higher RDW values are associated with decreased red blood cell deformability, which can impair microcirculation and cause tissue hypoxia 7
• RDW above 14.0% shows a threshold effect with significantly decreased RBC deformability 7
• Elevated RDW reflects underlying metabolic derangements including oxidative stress, inflammation, and impaired erythropoiesis 8

Critical Clinical Pitfalls to Avoid

• Never empirically treat with iron without confirming iron deficiency—this can cause iron overload in thalassemia patients 2
• Do not overlook combined deficiencies (iron plus B12 or folate), which may result in normal MCV despite abnormal RDW 2
• Always interpret RDW alongside other hematologic parameters including hemoglobin, MCV, and other red cell indices 1
• RDW values can vary between laboratory instruments, so interpret according to the specific laboratory's reference range 1

When to Investigate Further

• All adult men and post-menopausal women with confirmed iron deficiency anemia require endoscopic evaluation (upper and lower) to exclude malignancy, regardless of RDW values 2
• If clinical suspicion for iron deficiency exists despite normal RDW, additional testing with serum ferritin and transferrin saturation should be performed 1
• Consider hematology consultation for unexplained or refractory cases with abnormal RDW patterns 3