Differential Diagnosis for Decrease in MCV, MCH, and Increase in RBC

Single Most Likely Diagnosis

• Iron Deficiency Anemia: This is the most common cause of a decrease in Mean Corpuscular Volume (MCV) and Mean Corpuscular Hemoglobin (MCH), indicating microcytic hypochromic anemia. An increase in Red Blood Cell (RBC) count can occur as the body tries to compensate for the lack of oxygen delivery.

Other Likely Diagnoses

• Thalassemia: A genetic disorder affecting hemoglobin production, leading to microcytic anemia. Some forms of thalassemia can present with an increased RBC count due to the body's attempt to compensate for the ineffective erythropoiesis.
• Chronic Disease: Chronic diseases such as chronic kidney disease or rheumatoid arthritis can lead to a decrease in MCV and MCH due to chronic inflammation and/or decreased erythropoietin production. An increase in RBC count might not be directly related but can be seen in conditions with complex pathophysiology.
• Sideroblastic Anemia: A disorder where the bone marrow fails to utilize iron to create hemoglobin, resulting in microcytic hypochromic anemia. Some cases might show an increased RBC count.

Do Not Miss Diagnoses

• Polycythemia Vera: Although primarily characterized by an overproduction of all three blood cell types, in early stages or with certain mutations, it might present with an isolated increase in RBC count. The decrease in MCV and MCH could be relative or due to iron deficiency from frequent phlebotomies.
• Chronic Lung Disease: Conditions like chronic obstructive pulmonary disease (COPD) can lead to hypoxia, stimulating erythropoietin production and thus increasing RBC count. Secondary iron deficiency or other nutritional deficiencies might cause a decrease in MCV and MCH.

Rare Diagnoses

• Hemoglobinopathies (other than Thalassemia): Certain hemoglobinopathies can present with microcytic anemia and might have an increased RBC count due to the body's compensatory mechanisms.
• Myeloproliferative Neoplasms (other than Polycythemia Vera): Other myeloproliferative neoplasms, like essential thrombocythemia or primary myelofibrosis, can rarely present with an increase in RBC count and microcytic anemia, especially if there's associated iron deficiency or inflammation.