Diagnostic Approach to Elevated RDW and Low MCH

Primary Diagnostic Interpretation

The Centers for Disease Control and Prevention (CDC) recommends that a 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
Iron deficiency anemia is the leading diagnosis when RDW exceeds 14.0% combined with low MCH indicating hypochromic red cells, with the CDC specifically stating that a low MCV combined with high RDW (>14.0%) strongly indicates iron-deficiency anemia 1
An RDW of 18.0% reflects significant anisocytosis, which is characteristic of iron deficiency where both small, iron-poor cells and larger, older cells circulate simultaneously 1
The low MCH of 26.5 pg indicates hypochromia, meaning insufficient hemoglobin content per red cell, a hallmark of iron deficiency 1

The American College of Physicians recommends a minimum workup that must include serum ferritin (<30 μg/L confirms iron deficiency in absence of inflammation; <100 μg/L when inflammation present) 2
Transferrin saturation (<16-20% supports iron deficiency) is also recommended 1
C-reactive protein (to assess for concurrent inflammation) should be included in the workup 2
A complete blood count with MCV to confirm microcytosis is necessary 2
A reticulocyte count to assess bone marrow response is also recommended 2

Thalassemia minor typically presents with low MCV/MCH but normal or only mildly elevated RDW, making this diagnosis less likely with an RDW of 18.0% 1
Combined iron deficiency with folate or B12 deficiency can produce high RDW with variable MCH 2
Inflammatory conditions can elevate RDW while causing low MCH through impaired iron utilization 2
Platinum-based chemotherapy drugs can elevate RDW through eryptosis and accelerated red cell turnover 3

The degree of RDW elevation correlates with severity of iron deficiency, with research showing RDW demonstrates an inverse relationship with hemoglobin levels and transferrin saturation in iron deficiency 1

Do not assume thalassemia trait based solely on low MCH—the markedly elevated RDW of 18.0% makes iron deficiency far more likely 1
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

If iron deficiency is confirmed, initiate iron supplementation (oral or intravenous based on severity and tolerance) 1, 2
Investigate potential sources of blood loss, particularly gastrointestinal 1
Monitor response with repeat CBC in 4-8 weeks 3
RDW should normalize as iron stores replete and red cell production becomes more uniform 2