What does Mean Corpuscular Hemoglobin (MCH) indicate in a patient with anemia or other blood disorders?

What MCH Indicates

Mean Corpuscular Hemoglobin (MCH) measures the average amount of hemoglobin per red blood cell and is a reliable marker for iron deficiency, decreasing in both absolute and functional iron deficiency states. 1

Primary Clinical Significance

MCH is a marker of hypochromia (reduced hemoglobin content in red cells) and provides critical information about iron status and hemoglobin synthesis disorders. 1

• MCH is more reliable than MCV for detecting iron deficiency because it is less dependent on storage conditions and the specific counting machine used. 1
• Low MCH indicates hypochromia, which occurs primarily in iron deficiency but also in inherited disorders of hemoglobin synthesis, thalassemia, and sideroblastic anemia. 1
• MCH combined with low ferritin definitively confirms iron deficiency, making it a valuable diagnostic pairing. 2

Interpretation in Anemia Evaluation

When MCH is Low (Hypochromia)

Low MCH suggests the following conditions in order of clinical importance:

• Iron deficiency (absolute or functional) - the most common cause requiring investigation for blood loss, particularly gastrointestinal bleeding in non-menstruating patients. 1
• Inherited hemoglobin synthesis disorders including thalassemia (where MCV is typically reduced out of proportion to anemia) and sickle cell disease variants. 1
• Vitamin B12 or folate deficiency - though these more commonly cause macrocytosis, they can present with low MCH when combined with iron deficiency. 1
• Chronic disease/inflammation - though this typically causes normochromic anemia, functional iron deficiency can develop. 1

Critical Diagnostic Pitfall

In combined deficiencies (iron + vitamin B12), MCH remains low while MCV may appear falsely normal because macrocytic changes from B12 deficiency mask microcytic changes from iron deficiency. 2 This creates a dangerous scenario where:

• Normal MCV does not exclude either deficiency. 2
• MCH remains a more sensitive indicator of iron deficiency in this mixed picture. 1
• Red cell distribution width (RDW) will be elevated, indicating mixed cell populations. 2

Clinical Application Algorithm

When encountering low MCH, follow this systematic approach:

1. Check serum ferritin first - ferritin <15 μg/L confirms iron deficiency in the absence of inflammation; ferritin <30 μg/L is diagnostic without inflammation; ferritin up to 45-100 μg/L may still indicate iron deficiency when inflammation is present. 1, 2

2. Assess for inflammation - check CRP or inflammatory markers, as ferritin interpretation changes dramatically with inflammation. 2

3. Evaluate transferrin saturation - particularly important when inflammation is present, as it is less affected by acute phase response than ferritin. 1

4. Consider hemoglobin electrophoresis if microcytosis with normal iron studies, especially in patients of Mediterranean, African, or Southeast Asian descent to rule out thalassemia. 1

5. Check vitamin B12 and folate levels if RDW is elevated or clinical suspicion exists for combined deficiency. 2

Limitations and False Results

MCH can be falsely elevated by:

• Cold agglutination requiring 37°C water bath correction. 3
• Lipemic samples requiring plasma exchange. 3
• Hemolytic conditions where MCHC (not MCH specifically) may be spuriously elevated beyond physiologic limits. 4

MCH has reduced sensitivity in:

• Neonatal alpha-thalassemia screening, where overlap with normal values is substantial. 5
• Chronic disease states where normochromic anemia predominates. 1

Practical Clinical Value

MCH is most useful when:

• Screening for iron deficiency in anemia workup, where it may be more sensitive than MCV alone. 1
• Evaluating response to iron therapy - MCH should normalize with adequate treatment. 2
• Distinguishing iron deficiency from other causes of microcytosis when combined with ferritin and transferrin saturation. 1