What does an elevated Red Cell Distribution Width (RDW) indicate in a patient with Iron Deficiency Anemia (IDA)?

What RDW Means in Iron Deficiency Anemia

An elevated RDW (>14%) in iron deficiency anemia reflects anisocytosis—the presence of red blood cells of varying sizes—which is the hallmark morphologic feature distinguishing IDA from other microcytic anemias like thalassemia trait. 1

Pathophysiologic Significance

The elevated RDW in IDA indicates a heterogeneous red cell population where newly produced microcytic cells (due to iron deficiency) circulate alongside older normocytic cells, creating size variation that the automated analyzer detects as increased distribution width. 2, 1

• RDW rises early in iron deficiency, often before MCV drops, making it valuable for detecting latent iron deficiency when MCV, MCH, and MCHC remain normal. 3, 4
• In prelatent and latent stages of iron deficiency, RDW increases (typically >14%) while MCV stays normal, providing an early diagnostic clue. 3, 4
• RDW correlates inversely with hemoglobin levels and transferrin saturation in IDA—the more severe the iron deficiency, the higher the RDW. 5

Diagnostic Utility

The combination of high RDW (>14%) with low MCH and low MCHC is highly suggestive of iron deficiency anemia rather than thalassemia trait, which typically presents with normal or low RDW despite microcytosis. 1

Key Diagnostic Patterns:

• IDA: Elevated RDW (typically 15-20%) + low MCV + low ferritin 2, 5
• Thalassemia trait: Normal or low RDW (usually <17.1%) + low MCV + normal ferritin 5, 6
• Combined deficiency (iron + folate/B12): Elevated RDW + normal MCV (the deficiencies mask each other's effect on MCV but RDW remains high) 2, 1

Performance Characteristics:

• RDW has 82.3% sensitivity and 97.4% specificity for detecting iron deficiency in pregnant women, superior to MCV (29.2% sensitivity) in early stages. 3
• RDW >17.1% strongly suggests IDA and argues against thalassemia trait. 5
• In established IDA, 90% of patients have elevated RDW (>15%), compared to only 32% in anemia of chronic disease. 6

Clinical Algorithm

When you encounter elevated RDW in suspected IDA:

1. Confirm iron deficiency with serum ferritin (<12 μg/L diagnostic; <30 μg/L without inflammation; <100 μg/L with inflammation). 2, 1
2. Check transferrin saturation (<30% supports diagnosis). 2, 1
3. Measure CRP to identify concurrent inflammation that may elevate ferritin. 1
4. Do not assume thalassemia based on ethnicity alone—high RDW strongly favors iron deficiency over thalassemia trait. 2, 1

Critical Pitfalls

• Do not overlook combined deficiencies: Iron deficiency can coexist with folate or B12 deficiency, which normalizes MCV but maintains elevated RDW, potentially masking the diagnosis if you rely on MCV alone. 2, 1
• Do not skip gastrointestinal investigation in adult men and post-menopausal women with confirmed IDA—perform both upper endoscopy (with small bowel biopsies for celiac disease) and lower endoscopy to exclude malignancy, regardless of symptom presence. 2, 1
• Do not attribute IDA to dietary causes without investigation—GI blood loss from NSAID use, colorectal cancer, gastric cancer, or angiodysplasia is the most common cause requiring exclusion. 2, 1
• RDW alone cannot definitively distinguish IDA from thalassemia trait (66% of beta-thalassemia trait patients have elevated RDW), so always confirm with ferritin and consider hemoglobin electrophoresis when diagnostic uncertainty persists. 6