Anemia for USMLE Step 2
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Anemia for the USMLE Step 2 Exam
Overview of Anemia
- Definition:
- Anemia is defined as a decrease in hemoglobin (Hb) or hematocrit (Hct), resulting in reduced oxygen-carrying capacity.
- Diagnostic Thresholds:
- Men: Hb <13.5 g/dL or Hct <41%
- Women: Hb <12 g/dL or Hct <36%
- Classification by Mean Corpuscular Volume (MCV):
- Microcytic (MCV <80 fL): Typically due to iron deficiency, thalassemia, or anemia of chronic disease (ACD).
- Normocytic (MCV 80–100 fL): Commonly seen in ACD, hemolytic anemia, or acute blood loss.
- Macrocytic (MCV >100 fL): Often from vitamin B12 or folate deficiency, alcohol use, or liver disease.
Microcytic Anemias
- Iron Deficiency Anemia:
- Etiology: Due to chronic blood loss (e.g., gastrointestinal bleeding, menorrhagia), inadequate intake, or malabsorption.
- Pathophysiology: Iron is essential for hemoglobin synthesis; deficiency causes hypochromic, microcytic RBCs.
- Diagnosis: Low serum iron and ferritin, high total iron-binding capacity (TIBC).
- Treatment: Iron supplementation and addressing underlying causes.
- Thalassemia:
- Etiology: Genetic disorder reducing alpha or beta globin synthesis.
- Diagnosis: Microcytic anemia with normal or high RBC count; confirmed with hemoglobin electrophoresis.
- Treatment: Supportive care; transfusions for severe cases and iron chelation for transfusion-related iron overload.
- Anemia of Chronic Disease (ACD):
- Etiology: Chronic inflammation, infection, malignancy.
- Pathophysiology: Inflammatory cytokines limit iron availability and reduce erythropoiesis.
- Diagnosis: Low serum iron, low TIBC, normal or elevated ferritin.
- Treatment: Manage underlying condition; erythropoiesis-stimulating agents (ESAs) in specific cases.
Normocytic Anemias
- Hemolytic Anemia:
- Etiology: Can be inherited (e.g., hereditary spherocytosis, G6PD deficiency) or acquired (e.g., autoimmune hemolytic anemia).
- Diagnosis: Elevated reticulocyte count, indirect bilirubin, low haptoglobin, positive direct antiglobulin test (DAT) in autoimmune cases.
- Treatment: Varies by cause; corticosteroids for autoimmune hemolysis, avoidance of triggers in G6PD deficiency.
- Aplastic Anemia:
- Etiology: Bone marrow failure due to autoimmune, viral, or drug-induced causes.
- Diagnosis: Normocytic anemia with pancytopenia and hypocellular bone marrow on biopsy.
- Treatment: Immunosuppressive therapy or bone marrow transplant.
- Acute Blood Loss:
- Etiology: Trauma, surgery, or gastrointestinal bleeding.
- Diagnosis: Normocytic anemia with elevated reticulocyte count; clinical evidence of blood loss.
- Treatment: Hemodynamic support, transfusions if needed.
Macrocytic Anemias
- Vitamin B12 Deficiency:
- Etiology: Malabsorption (e.g., pernicious anemia, gastric surgery) or dietary insufficiency.
- Diagnosis: Low serum B12, elevated methylmalonic acid (MMA) and homocysteine levels.
- Treatment: B12 supplementation (oral or parenteral) and management of malabsorption causes.
- Folate Deficiency:
- Etiology: Malnutrition, alcoholism, pregnancy, or certain medications (e.g., methotrexate).
- Diagnosis: Low serum folate, elevated homocysteine, normal MMA.
- Treatment: Oral folate supplementation and dietary changes.
- Liver Disease and Alcohol Use:
- Pathophysiology: Alcohol affects folate metabolism, and liver disease disrupts RBC maturation.
- Diagnosis: Macrocytic anemia with elevated liver enzymes and alcohol use history.
- Treatment: Alcohol cessation, folate supplementation if deficient.
Diagnostic Workup
- Complete Blood Count (CBC):
- Assess Hb, Hct, MCV, and red cell distribution width (RDW).
- Iron Studies:
- Useful for distinguishing iron deficiency anemia from ACD.
- Peripheral Blood Smear:
- Identifies characteristic RBC morphologies (e.g., schistocytes in hemolysis, hypersegmented neutrophils in B12 deficiency).
- Reticulocyte Count:
- Elevated in hemolytic or blood loss anemia, decreased in production-related anemias.
Key Points
- Anemia is classified by MCV to guide the diagnostic approach and treatment strategy.
- Microcytic anemia is commonly caused by iron deficiency, thalassemia, or ACD; iron studies are essential for diagnosis.
- Normocytic anemia can result from hemolysis, acute blood loss, or bone marrow suppression, with reticulocyte count aiding in differentiation.
- Macrocytic anemia is typically due to vitamin B12 or folate deficiency; specific testing for MMA and homocysteine helps confirm diagnosis.
- Management includes correcting deficiencies, transfusions in severe cases, and treating underlying causes.