Anemia for USMLE Step 1

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Anemia for the USMLE Step 1 Exam

Overview of Anemia

Anemia is a condition marked by low hemoglobin (Hb) or hematocrit (Hct) levels, resulting in decreased oxygen-carrying capacity.
Criteria for anemia:
Men: Hb <13.5 g/dL or Hct <41%
Women: Hb <12 g/dL or Hct <36%

Microcytic (MCV <80 fL): Often caused by iron deficiency, thalassemia, or anemia of chronic disease (ACD).
Normocytic (MCV 80–100 fL): Seen in ACD, hemolytic anemia, or acute blood loss.
Macrocytic (MCV >100 fL): Usually due to vitamin B12 or folate deficiency, alcohol use, or liver disease.

Microcytic Anemias

Etiology: Due to inadequate iron intake, malabsorption (e.g., celiac disease), or chronic blood loss.
Pathophysiology: Iron deficiency leads to low hemoglobin synthesis and hypochromic, microcytic RBCs.
Diagnosis: Low serum iron and ferritin, high total iron-binding capacity (TIBC).
Treatment: Iron supplementation and addressing the underlying cause.

Etiology: Genetic disorder reducing synthesis of alpha or beta globin chains.
Diagnosis: Microcytic anemia with normal iron levels; hemoglobin electrophoresis shows abnormal hemoglobin types.
Treatment: Supportive care, blood transfusions if severe, and iron chelation for transfusion-related iron overload.

Etiology: Associated with chronic infections, cancer, autoimmune diseases.
Pathophysiology: Inflammatory cytokines impair iron metabolism and RBC production.
Diagnosis: Low serum iron and TIBC, normal or elevated ferritin.
Treatment: Manage underlying disease; erythropoiesis-stimulating agents (ESAs) in specific cases.

Normocytic Anemias

Etiology: Includes hereditary (e.g., hereditary spherocytosis, G6PD deficiency) and acquired causes (e.g., autoimmune hemolysis).
Diagnosis: Elevated reticulocyte count, increased indirect bilirubin, low haptoglobin.
Treatment: Specific to cause; for autoimmune, corticosteroids or immunosuppressants may be required.

Etiology: Caused by bone marrow failure due to autoimmune, viral, or drug-induced factors.
Diagnosis: Pancytopenia with hypocellular bone marrow.
Treatment: Immunosuppressive therapy or bone marrow transplant in severe cases.

Macrocytic Anemias

Etiology: Malabsorption (e.g., pernicious anemia, gastric surgery) or dietary insufficiency.
Pathophysiology: B12 is needed for DNA synthesis; deficiency causes ineffective erythropoiesis and macrocytosis.
Diagnosis: Low serum B12, elevated methylmalonic acid (MMA) and homocysteine levels.
Treatment: B12 supplementation (oral or parenteral).

Etiology: Common in alcoholism, malnutrition, and certain medications (e.g., methotrexate).
Diagnosis: Low serum folate, elevated homocysteine, normal MMA.
Treatment: Oral folate supplementation and dietary adjustments.

Diagnostic Workup

Serum iron, ferritin, TIBC, and transferrin saturation help distinguish iron deficiency from ACD.

Assesses bone marrow response; elevated in hemolytic anemias, decreased in production anemias.

Identifies specific RBC morphologies (e.g., schistocytes in hemolysis, hypersegmented neutrophils in B12 deficiency).

Key Points

Anemia is classified by RBC size (microcytic, normocytic, macrocytic) to guide diagnosis and treatment.
Microcytic anemia causes include iron deficiency, thalassemia, and chronic disease, often requiring iron studies for diagnosis.
Normocytic anemia can result from blood loss, hemolysis, or bone marrow disorders, with reticulocyte count aiding in differentiation.
Macrocytic anemia commonly results from vitamin B12 or folate deficiency, with specific testing for MMA and homocysteine to confirm.
Treatment varies based on etiology, with iron and vitamin supplementation for deficiencies, immunosuppressive therapy for aplastic anemia, and blood transfusions for severe cases.