RDW

Red Cell Distribution Width

Complete Blood Count

What is Red Cell Distribution Width?

Red cell distribution width (RDW) measures the degree of variation in the size (volume) of your red blood cells, a phenomenon known as anisocytosis. It is expressed as a percentage and is calculated from the standard deviation of red blood cell volumes divided by the MCV, multiplied by 100. A normal RDW means most red blood cells are similar in size, while a high RDW indicates significant variation—a mix of smaller and larger cells circulating together. RDW is automatically calculated and reported as part of a standard CBC.

RDW has traditionally been used alongside MCV to classify and narrow the differential diagnosis of anemias. However, research over the past two decades has revealed that RDW has broader clinical significance than initially appreciated. Elevated RDW has been independently associated with increased mortality in general populations and in patients with cardiovascular disease, heart failure, cancer, and critical illness. The mechanisms likely involve the underlying conditions that produce anisocytosis—oxidative stress, inflammation, nutritional deficiencies, and impaired erythropoiesis—all of which reflect systemic health status.

Why It Matters

RDW is most valuable when interpreted alongside MCV for anemia classification. Iron deficiency anemia produces a high RDW because ongoing iron depletion leads to progressively smaller cells, creating a mix of normal and microcytic cells. Thalassemia trait, by contrast, produces uniformly small cells with a normal RDW. This distinction—high RDW versus normal RDW with low MCV—is one of the most useful patterns in hematologic diagnosis. Beyond anemia, elevated RDW has emerged as a powerful prognostic marker. Studies have consistently shown that high RDW predicts poor outcomes in heart failure, acute coronary syndromes, sepsis, and general hospitalized populations, making it a surprisingly informative and inexpensive test.

Normal Reference Ranges

GroupRangeUnit
Adults (RDW-CV)11.5–14.5%
Adults (RDW-SD)39–46fL

Reference ranges may vary by laboratory. Always compare results to the ranges provided by your testing facility.

What High RDW Levels Mean

Common Causes

  • Iron deficiency anemia (most common cause)
  • Vitamin B12 or folate deficiency
  • Mixed nutritional deficiencies (iron + B12, causing dimorphic picture)
  • Myelodysplastic syndrome
  • Hemolytic anemias (sickle cell disease, autoimmune hemolysis)

Possible Symptoms

  • Fatigue and weakness (from underlying anemia)
  • Shortness of breath on exertion
  • Pallor
  • Symptoms specific to the underlying deficiency or disease

What to do: Elevated RDW should prompt evaluation in the context of the MCV. High RDW with low MCV: check iron studies for iron deficiency. High RDW with high MCV: check B12, folate, liver function. High RDW with normal MCV: consider early iron or B12 deficiency (before MCV changes), mixed deficiency, or recent blood transfusion. Persistent high RDW without nutritional deficiency may warrant peripheral blood smear and potentially bone marrow evaluation to rule out myelodysplasia. Address the underlying cause—supplementation for deficiencies, treatment of hemolysis or bone marrow disorders.

What Low RDW Levels Mean

Common Causes

  • Normal variation—low RDW generally indicates uniform red blood cell size
  • Thalassemia trait (uniformly small cells)
  • Anemia of chronic disease (uniformly normocytic cells)
  • Recent blood transfusion with well-matched blood (transient)

Possible Symptoms

  • Low RDW itself does not cause symptoms
  • Symptoms, if present, relate to any underlying anemia or condition
  • Often found incidentally on routine CBC

What to do: A low or normal RDW is generally reassuring and does not require specific investigation. If anemia is present with a low RDW and low MCV, thalassemia trait should be considered and hemoglobin electrophoresis may be appropriate. If the RDW is low with a normal MCV and normal hemoglobin, no further workup is needed. A low RDW is not a clinical concern in itself.

When Is RDW Testing Recommended?

  • As part of a routine CBC (automatically calculated)
  • When classifying the type of anemia alongside MCV
  • When distinguishing iron deficiency from thalassemia trait
  • When assessing prognosis in cardiovascular disease or critical illness

Frequently Asked Questions

Both iron deficiency anemia and thalassemia trait cause microcytosis (low MCV), but they affect RDW differently. In iron deficiency, depletion is gradual—as iron stores fall, newly produced cells become progressively smaller while older, normal-sized cells remain in circulation. This mix of small and normal cells produces a high RDW (>14.5%). In thalassemia trait, the genetic defect in globin chain production causes all red blood cells to be uniformly small from the moment of production, resulting in a normal RDW. This distinction has a sensitivity of approximately 85–90% and is a useful initial screening tool, though iron studies and hemoglobin electrophoresis are needed for definitive diagnosis.
The association between elevated RDW and poor outcomes has been one of the most replicated findings in clinical hematology research. The likely explanation is that RDW serves as a composite biomarker reflecting multiple pathological processes. Elevated RDW can result from nutritional deficiency, chronic inflammation (inflammatory cytokines impair erythropoiesis), oxidative stress (which damages red blood cell membranes and shortens their lifespan), impaired renal function (reduced EPO), and neurohumoral activation (as in heart failure). These are all processes that independently predict poor outcomes. RDW captures their cumulative effect on erythropoiesis, making it a sensitive, nonspecific marker of overall physiological stress and systemic illness.
A dimorphic blood picture occurs when two distinct populations of red blood cells coexist—for example, a mixture of microcytic and macrocytic cells. This produces a characteristically high RDW with a potentially normal MCV (because the average of small and large cells can be normal). The most common cause is combined iron and B12/folate deficiency, where iron deficiency produces microcytes and B12/folate deficiency produces macrocytes simultaneously. Other causes include recent blood transfusion (donor cells mixed with patient cells), early response to treatment of nutritional deficiency (new normal cells mixing with old abnormal cells), and sideroblastic anemia. A peripheral blood smear is particularly helpful in identifying dimorphic populations that the MCV alone would miss.

Related Biomarkers

HgbHemoglobinMCVMean Corpuscular VolumeFeSerum IronFerritinFerritin

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Medical Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Reference ranges may vary between laboratories. Always consult your healthcare provider for interpretation of your specific test results.

Disclaimer: SymptomGPT is not a medical diagnosis tool and does not provide medical advice. Always consult a qualified healthcare professional. If you are experiencing a medical emergency, call 911 immediately.