What does a normal hemoglobin level with a low red blood cell distribution width indicate?

Normal Hemoglobin with Low RDW: Clinical Interpretation

A normal hemoglobin level with a low RDW (≤14.0%) indicates uniform red blood cell size and essentially rules out iron deficiency anemia, suggesting either a normal state, thalassemia trait (if microcytosis is present), or early anemia of chronic disease. 1, 2

Key Diagnostic Principles

Low RDW indicates homogeneous red blood cell populations, which is the opposite pattern seen in iron deficiency anemia where RDW is characteristically elevated (>14.0%) due to heterogeneous cell sizes. 3, 1

What This Pattern Rules Out

• Iron deficiency anemia is highly unlikely with low RDW, as iron deficiency typically causes RDW >14.0% due to variable red cell sizes during progressive iron depletion. 3, 2, 4
• Studies demonstrate that iron deficiency anemia patients have mean RDW of 18-20%, significantly higher than the normal reference of ≤14.0%. 4, 5

What This Pattern Suggests

The clinical significance depends on whether microcytosis (low MCV) is present:

If MCV is Low (Microcytic)

• Thalassemia trait is the most likely diagnosis when low MCV combines with low/normal RDW, as thalassemia produces uniformly small red cells. 3, 2, 6
• The combination of low MCV + low RDW has significantly better discriminatory value for thalassemia trait versus iron deficiency than other indices. 6
• Do not initiate iron supplementation based on microcytosis alone without confirming iron deficiency via serum ferritin, as this can cause iron overload in thalassemia patients. 2

If MCV is Normal (Normocytic)

• This likely represents a normal state or very early anemia of chronic disease. 3, 2
• The normal hemoglobin makes significant pathology unlikely in this scenario. 3

Essential Workup When Uncertainty Exists

Even with normal hemoglobin, if clinical suspicion warrants investigation, the minimum workup should include: 3, 1

• Serum ferritin (<30 μg/L without inflammation or <100 μg/L with inflammation indicates iron deficiency) 3, 2
• Transferrin saturation (<30% supports iron deficiency) 2
• C-reactive protein (CRP) to assess for inflammation 3, 2
• Hemoglobin electrophoresis if thalassemia is suspected based on ethnicity and microcytosis 3, 2

Critical Clinical Pitfalls to Avoid

• Never assume iron deficiency based solely on microcytosis—low RDW makes iron deficiency significantly less likely, not more likely. 1, 2
• Do not empirically treat with iron without laboratory confirmation of iron deficiency, particularly in patients from ethnic backgrounds with higher thalassemia prevalence. 3, 2
• Recognize that combined deficiencies (iron plus folate or B12) can result in normal MCV despite iron deficiency, though RDW would typically be elevated in this scenario. 3, 1

When to Investigate Further

If anemia develops later (hemoglobin drops below normal), investigation becomes necessary: 3

• Adult men and post-menopausal women with confirmed iron deficiency require both upper and lower gastrointestinal endoscopy to exclude malignancy, regardless of RDW values. 3, 2
• Even mild anemia warrants investigation, as severity does not correlate with likelihood of serious underlying disease. 3, 2