Anemia for USMLE Step 3
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Anemia for the USMLE Step 3 Exam
Overview of Anemia
- Definition:
- Anemia is a reduction in hemoglobin (Hb) or hematocrit (Hct) levels, lowering the blood’s oxygen-carrying capacity.
- Diagnostic cutoffs:
- 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): Commonly caused by iron deficiency, thalassemia, and anemia of chronic disease (ACD).
- Normocytic (MCV 80–100 fL): Often due to ACD, hemolytic anemia, or acute blood loss.
- Macrocytic (MCV >100 fL): Frequently associated with vitamin B12 or folate deficiency, alcohol use, or liver disease.
Microcytic Anemias
- Iron Deficiency Anemia:
- Etiology: Typically caused by chronic blood loss (e.g., gastrointestinal bleeding), poor dietary intake, or malabsorption.
- Diagnosis: Low serum iron, ferritin, high total iron-binding capacity (TIBC).
- Treatment: Iron supplementation (oral or IV) and treatment of the underlying cause.
- Thalassemia:
- Etiology: Genetic mutation leading to reduced alpha or beta globin chain production.
- Diagnosis: Microcytic anemia with normal or high RBC count; confirmed by hemoglobin electrophoresis.
- Treatment: Mild cases require no treatment; severe cases may need transfusions and iron chelation to prevent iron overload.
- Anemia of Chronic Disease (ACD):
- Etiology: Associated with chronic inflammatory diseases, malignancies, or infections.
- Diagnosis: Low serum iron, low TIBC, and normal or elevated ferritin.
- Treatment: Management of underlying disease; erythropoiesis-stimulating agents (ESAs) if indicated.
Normocytic Anemias
- Hemolytic Anemia:
- Etiology: Includes hereditary causes (e.g., spherocytosis, G6PD deficiency) and acquired causes (e.g., autoimmune hemolytic anemia).
- Diagnosis: Elevated reticulocyte count, increased indirect bilirubin, low haptoglobin, positive direct antiglobulin test (DAT) in autoimmune cases.
- Treatment: Based on etiology; corticosteroids for autoimmune hemolysis, avoidance of triggers in G6PD deficiency.
- Aplastic Anemia:
- Etiology: Bone marrow failure due to autoimmune causes, viral infections, or drugs.
- Diagnosis: Pancytopenia with a hypocellular bone marrow biopsy.
- Treatment: Immunosuppressive therapy or bone marrow transplant.
- Acute Blood Loss:
- Etiology: Due to trauma, gastrointestinal bleeding, or surgery.
- Diagnosis: Normocytic anemia with elevated reticulocyte count; clinical history of bleeding.
- Treatment: Stabilization, blood transfusions if necessary.
Macrocytic Anemias
- Vitamin B12 Deficiency:
- Etiology: Caused by malabsorption (e.g., pernicious anemia, gastric bypass) or inadequate intake.
- Diagnosis: Macrocytic anemia with low serum B12, elevated methylmalonic acid (MMA) and homocysteine levels.
- Treatment: B12 supplementation (oral or parenteral); manage underlying cause of malabsorption.
- Folate Deficiency:
- Etiology: Commonly associated with poor diet, alcoholism, pregnancy, and medications (e.g., methotrexate).
- Diagnosis: Low serum folate, elevated homocysteine, normal MMA.
- Treatment: Oral folate supplementation; dietary changes to increase folate intake.
- Alcohol-Related and Liver Disease:
- Pathophysiology: Alcohol affects folate metabolism, leading to macrocytosis.
- Diagnosis: Macrocytic anemia with elevated liver enzymes; history of alcohol use.
- Treatment: Alcohol cessation, folate supplementation if deficient.
Diagnostic Approach
- Complete Blood Count (CBC):
- Evaluates Hb, Hct, MCV, and red cell distribution width (RDW).
- Peripheral Blood Smear:
- Identifies characteristic RBC morphologies (e.g., schistocytes in hemolysis, hypersegmented neutrophils in B12 deficiency).
- Iron Studies:
- Helps distinguish iron deficiency from ACD.
- Reticulocyte Count:
- High in hemolysis or blood loss, low in production anemias.
Key Points
- Anemia is classified by MCV into microcytic, normocytic, and macrocytic, guiding further evaluation and treatment.
- Microcytic anemia causes include iron deficiency, thalassemia, and ACD; diagnostic workup includes iron studies.
- Normocytic anemia may arise from acute blood loss, hemolysis, or bone marrow failure; a reticulocyte count helps identify the etiology.
- Macrocytic anemia commonly results from vitamin B12 or folate deficiency; serum MMA and homocysteine are helpful for differentiation.
- Treatment is etiology-specific, including iron or vitamin supplementation, transfusions in severe cases, and addressing underlying conditions.