Mean Corpuscular Hemoglobin (MCH) Significance in CBC
A reduced MCH indicates hypochromia and is a reliable marker of iron deficiency, being more sensitive and less machine-dependent than MCV, while an elevated MCH suggests hemochromatosis or other conditions with increased erythrocyte hemoglobin content. 1
Understanding MCH as a Diagnostic Marker
Low MCH (Hypochromia)
MCH is probably a more reliable marker of iron deficiency than MCV because it is less dependent on storage conditions and the specific counting machine used. 1 A reduction in MCH occurs in both absolute and functional iron deficiency, making it particularly valuable in clinical practice. 1
Key clinical points about low MCH:
- Low MCH indicates hypochromia (reduced hemoglobin concentration in red cells) and typically accompanies iron deficiency anemia. 1
- MCH may be more sensitive for detecting iron deficiency than MCV alone. 1
- Both microcytosis and hypochromia lose sensitivity for iron deficiency in the presence of chronic disease, thalassemia, or vitamin B12/folate deficiency. 1
Differential Diagnosis of Low MCH
The specificity of MCH for iron deficiency is limited, as hypochromia also occurs in: 1
- Hemoglobinopathies (such as thalassemia, where MCV is typically reduced out of proportion to the level of anemia) 1
- Sideroblastic anemia 1
- Some cases of anemia of chronic disease 1
Critical pitfall to avoid: In patients with microcytosis/hypochromia and normal iron studies, particularly those with appropriate ethnic background, Hb electrophoresis is recommended to prevent unnecessary GI investigation and identify underlying hemoglobinopathies. 1
High MCH
Elevated MCH values occur in hemochromatosis due to increased iron uptake and hemoglobin synthesis by immature erythroid cells. 2 In hemochromatosis probands, mean MCH values are significantly higher than in normal control subjects, even after iron depletion. 2
Other causes of elevated MCH include:
- Macrocytic anemias (vitamin B12 or folate deficiency) 1
- Certain medications (hydroxyurea, diphenytoin) 1
- Myelodysplastic syndromes 1
- Alcoholism 1
Clinical Approach to Abnormal MCH
When MCH is Low
Confirm iron deficiency with serum ferritin (the single most useful marker), along with transferrin saturation if false-normal ferritin is suspected due to inflammation. 1
- Serum ferritin <15 μg/L is highly specific for iron deficiency (specificity 0.99) 1
- Ferritin <30 μg/L generally indicates low body iron stores 1
- In the presence of inflammation, a ferritin cut-off of 45 μg/L provides optimal sensitivity/specificity trade-off 1
- Ferritin >150 μg/L is unlikely to occur with absolute iron deficiency, even with inflammation 1
When MCH is Normal or High
MCV should be considered alongside MCH for complete morphologic assessment. 1
- Normocytic with normal MCH: Consider hemorrhage, hemolysis, bone marrow failure, anemia of chronic inflammation, or renal insufficiency—check reticulocyte count. 1
- Macrocytic with high MCH: Check vitamin B12 and folate levels; consider medication review and assess for MDS. 1
Important Caveats
MCH is considered a late marker of iron deficiency in chronic kidney disease patients, making it less useful than ferritin and transferrin saturation for early detection. 1
Technical considerations: MCH values can be affected by sample storage time and temperature, though less so than MCV. 3 For optimal accuracy, CBC testing should be performed as soon as possible after blood collection. 3