Causes of RDW 18.0% and MCH 26.5
The combination of elevated RDW (18.0%) and low MCH (26.5 pg) most strongly indicates iron deficiency anemia, which should be confirmed with serum ferritin and transferrin saturation testing. 1, 2
Primary Diagnostic Interpretation
Iron deficiency anemia is the leading diagnosis when RDW exceeds 14.0% (your value of 18.0% is markedly elevated) combined with low MCH indicating hypochromic red cells. 1, 2 The CDC specifically states that a low MCV combined with high RDW (>14.0%) strongly indicates iron-deficiency anemia, and since MCH parallels MCV in microcytic conditions, this same logic applies. 1
- An RDW of 18.0% reflects significant anisocytosis (variation in red cell size), which is characteristic of iron deficiency where both small, iron-poor cells and larger, older cells circulate simultaneously 1, 3
- The low MCH of 26.5 pg (normal ~27-33 pg) indicates hypochromia, meaning insufficient hemoglobin content per red cell, a hallmark of iron deficiency 1
- Research demonstrates that RDW values above 17.1% strongly suggest iron deficiency anemia, and your value of 18.0% exceeds this threshold 3
Confirmatory Testing Required
Minimum workup must include: 2
- Serum ferritin (<30 μg/L confirms iron deficiency in absence of inflammation; <100 μg/L when inflammation present)
- Transferrin saturation (<16-20% supports iron deficiency)
- C-reactive protein (to assess for concurrent inflammation)
- Complete blood count with MCV to confirm microcytosis
- Reticulocyte count to assess bone marrow response
Differential Diagnoses to Consider
Thalassemia Trait (Less Likely)
- Thalassemia minor typically presents with low MCV/MCH but normal or only mildly elevated RDW (usually ≤14.0-15.1%), making this diagnosis less likely with your RDW of 18.0% 1, 3
- If RDW were <17.1%, thalassemia would be more probable, but your elevated value argues against this 3
Mixed Deficiency States
- Combined iron deficiency with folate or B12 deficiency can produce high RDW with variable MCH, though typically MCH would be less uniformly low 2
- Inflammatory conditions can elevate RDW while causing low MCH through impaired iron utilization 2
Active Hemolysis or Hemoglobinopathies
- Sickle cell trait, sickle cell disease, or sickle-beta thalassemia can produce elevated RDW (highest values seen in sickle cell anemia) with low MCH 4
- However, these conditions typically present with additional clinical features and would require hemoglobin electrophoresis for diagnosis 4
Medication-Induced Changes
- Platinum-based chemotherapy drugs can elevate RDW through eryptosis and accelerated red cell turnover 5
- Long-term antibiotics (nafcillin, rifampin) may affect red cell parameters through CYP450 enzyme induction, though this is less common 5
Clinical Significance and Severity Assessment
The degree of RDW elevation correlates with severity of iron deficiency. 3 Research shows RDW demonstrates an inverse relationship with hemoglobin levels (r = -0.543) and transferrin saturation in iron deficiency—higher RDW indicates more severe depletion. 3
Common Pitfalls to Avoid
- Do not assume thalassemia trait based solely on low MCH—the markedly elevated RDW of 18.0% makes iron deficiency far more likely 1, 3
- Do not overlook concurrent inflammation—ferritin cutoffs differ when inflammation is present (<100 μg/L vs <30 μg/L) 2
- Do not delay investigation of blood loss sources—once iron deficiency is confirmed, gastrointestinal evaluation is essential, especially in adults 2
- Do not rely on capillary blood sampling without confirmation—improper technique can yield false low readings; venous sampling is preferred 1
Treatment Approach Once Confirmed
If iron deficiency is confirmed: 2
- Initiate iron supplementation (oral or intravenous based on severity and tolerance)
- Investigate potential sources of blood loss, particularly gastrointestinal
- Monitor response with repeat CBC in 4-8 weeks
- RDW should normalize as iron stores replete and red cell production becomes more uniform