What is a common cause of a mean corpuscular hemoglobin (MCH) of 33.1?

Common Causes of MCH 33.1

An elevated mean corpuscular hemoglobin (MCH) of 33.1 pg is most commonly caused by macrocytosis, with vitamin B12 deficiency, folate deficiency, and alcohol use being the most frequent underlying etiologies. This value exceeds the normal MCH range, which typically falls between 26-33 pg in adults.

Understanding MCH and Its Significance

MCH represents the average amount of hemoglobin contained in a single red blood cell. An elevated MCH (>33 pg) generally indicates macrocytosis, where red blood cells are larger than normal and contain more hemoglobin per cell 1.

Common causes of elevated MCH include:

• Vitamin B12 deficiency: Leads to impaired DNA synthesis and megaloblastic anemia with large RBCs containing increased hemoglobin 1
• Folate deficiency: Similar to B12 deficiency, causes megaloblastic anemia with macrocytic RBCs 1
• Alcohol abuse: Directly toxic to bone marrow, causing macrocytosis even without anemia 1
• Medications: Particularly chemotherapeutic agents, anticonvulsants, and thiopurines (azathioprine, 6-mercaptopurine) 1
• Liver disease: Affects red cell membrane composition, leading to macrocytosis 1
• Hypothyroidism: Slows metabolism and affects erythropoiesis 1
• Reticulocytosis: Increased immature RBCs (reticulocytes) are larger and have higher MCH 1

Clinical Approach to Elevated MCH

When evaluating a patient with MCH of 33.1, a systematic approach is essential:

1. Check other RBC indices:

   • MCV (mean corpuscular volume): Will typically be elevated (>100 fL) in true macrocytosis 1, 2
   • MCHC (mean corpuscular hemoglobin concentration): May be normal or slightly elevated 2
   • RDW (red cell distribution width): Often elevated in nutritional deficiencies 1

2. Evaluate for anemia:

   • If hemoglobin is low, consider megaloblastic anemia from B12/folate deficiency 1
   • If hemoglobin is normal, consider non-anemic causes like alcohol use or medications 1

3. Additional laboratory tests:

   • Serum B12 and folate levels 1
   • Liver function tests 1
   • Thyroid function tests 1
   • Reticulocyte count (elevated in hemolysis or blood loss) 1
   • Peripheral blood smear to evaluate RBC morphology 1

Clinical Significance and Pitfalls

An MCH of 33.1 requires careful interpretation as it may indicate underlying pathology that affects morbidity and mortality:

• Recent research demonstrates a U-shaped relationship between MCH and mortality risk, with values above 30.2 pg associated with increased all-cause mortality (adjusted HR 1.08,95% CI 1.04-1.12) 2

• Common pitfalls to avoid:

  • Failing to check B12 and folate levels in patients with elevated MCH 1
  • Overlooking alcohol use as a potential cause 1
  • Not considering medication effects, particularly in patients on chemotherapy or anticonvulsants 1
  • Missing concomitant iron deficiency, which can mask macrocytosis by lowering MCV 1

• Special considerations:

  • In patients with hemoglobinopathies, MCH interpretation may be complicated 3, 4
  • Genetic factors can influence MCH values, as demonstrated by genome-wide association studies 5

Monitoring and Follow-up

For patients with elevated MCH:

• Repeat complete blood count in 2-3 months after addressing the underlying cause 1
• Monitor response to therapy (e.g., B12/folate supplementation, alcohol cessation) 1
• Consider hematology consultation if MCH remains elevated despite addressing obvious causes 1