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Anemia for ABIM

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Anemia for the American Board of Internal Medicine Exam
Overview of Anemia
  • Definition:
    • Anemia is defined as a decrease in red blood cell (RBC) mass, often indicated by low hemoglobin (Hb) or hematocrit (Hct) levels.
    • Diagnostic thresholds for anemia:
    • Men: Hb <13.5 g/dL or Hct <41%
    • Women: Hb <12 g/dL or Hct <36%
  • Pathophysiology:
    • Anemia may result from decreased RBC production, increased RBC destruction, or blood loss.
    • RBC production is typically affected by bone marrow health, erythropoietin levels, and nutrient availability (iron, B12, folate).
Classification of Anemia
  • By RBC Size (MCV):
    • Microcytic (MCV <80 fL): Common causes include iron deficiency anemia, thalassemia, anemia of chronic disease (ACD), and sideroblastic anemia.
    • Normocytic (MCV 80–100 fL): Often seen in ACD, hemolytic anemia, and acute blood loss.
    • Macrocytic (MCV >100 fL): Often due to vitamin B12 or folate deficiency, liver disease, or alcohol use.
  • By Pathophysiology:
    • Decreased Production: Nutritional deficiencies, bone marrow disorders, chronic disease.
    • Increased Destruction (Hemolysis): Hereditary spherocytosis, G6PD deficiency, autoimmune hemolytic anemia.
    • Blood Loss: Gastrointestinal bleeding, menorrhagia, trauma.
Microcytic Anemias
  • Iron Deficiency Anemia:
    • Etiology: Most common cause of anemia; often due to blood loss (e.g., GI bleed, menstruation), inadequate intake, or malabsorption.
    • Pathophysiology: Iron is required for hemoglobin synthesis; deficiency leads to hypochromic, microcytic RBCs.
    • Diagnosis: Low serum iron, low ferritin, high total iron-binding capacity (TIBC), low transferrin saturation.
    • Treatment: Oral or IV iron supplementation; identify and address underlying cause.
  • Thalassemia:
    • Etiology: Genetic defect affecting globin chain synthesis, leading to reduced hemoglobin production.
    • Types: Alpha and beta thalassemia, with varying severity.
    • Diagnosis: Microcytic anemia with normal to high RBC count; confirmed by hemoglobin electrophoresis.
    • Treatment: Supportive care, transfusions if severe; chelation therapy for iron overload in transfusion-dependent patients.
  • Anemia of Chronic Disease (ACD):
    • Etiology: Often associated with chronic infections, inflammation, cancer, and autoimmune diseases.
    • Pathophysiology: Inflammatory cytokines inhibit erythropoiesis and iron metabolism.
    • Diagnosis: Normocytic or microcytic; low serum iron, low TIBC, normal or elevated ferritin.
    • Treatment: Manage underlying condition; erythropoiesis-stimulating agents (ESAs) in select cases.
Normocytic Anemias
  • Hemolytic Anemia:
    • Etiology: Includes hereditary causes (e.g., spherocytosis, G6PD deficiency) and acquired causes (e.g., autoimmune hemolysis, drug-induced).
    • Pathophysiology: Premature RBC destruction, leading to increased reticulocyte count.
    • Diagnosis: Elevated reticulocyte count, high indirect bilirubin, low haptoglobin, positive direct antiglobulin test (DAT) in autoimmune cases.
    • Treatment: Depends on etiology; steroids for autoimmune hemolysis, avoiding triggers in G6PD deficiency.
  • Aplastic Anemia:
    • Etiology: Bone marrow failure due to autoimmune, viral, or drug-induced causes.
    • Pathophysiology: Pancytopenia due to decreased hematopoiesis.
    • Diagnosis: Normocytic anemia with pancytopenia, hypocellular bone marrow biopsy.
    • Treatment: Immunosuppressive therapy, bone marrow transplant in severe cases.
  • Acute Blood Loss:
    • Etiology: Trauma, surgery, gastrointestinal bleeding.
    • Pathophysiology: Loss of RBCs leading to hypovolemia and reduced oxygen-carrying capacity.
    • Diagnosis: Normocytic anemia with elevated reticulocyte count; clinical evidence of blood loss.
    • Treatment: Hemodynamic stabilization, transfusions if needed.
Macrocytic Anemias
  • Vitamin B12 Deficiency:
    • Etiology: Often due to malabsorption (e.g., pernicious anemia, gastric bypass, Crohn’s disease) or inadequate intake.
    • Pathophysiology: B12 is crucial for DNA synthesis; deficiency results in ineffective erythropoiesis and macrocytosis.
    • Diagnosis: Macrocytic anemia with low serum B12, elevated methylmalonic acid (MMA), and homocysteine levels.
    • Treatment: B12 supplementation (oral or parenteral); treat underlying cause.
  • Folate Deficiency:
    • Etiology: Common in malnutrition, alcohol use, pregnancy, and certain medications (e.g., methotrexate).
    • Pathophysiology: Folate is needed for DNA synthesis; deficiency leads to macrocytic anemia.
    • Diagnosis: Low serum folate, elevated homocysteine, normal MMA.
    • Treatment: Oral folate supplementation, dietary changes.
  • Alcohol-Related and Liver Disease:
    • Pathophysiology: Alcohol disrupts folate metabolism and affects bone marrow, leading to macrocytosis.
    • Diagnosis: Macrocytic anemia with elevated liver enzymes and history of alcohol use.
    • Treatment: Alcohol cessation, folate supplementation if deficient.
Diagnostic Approach
  • Complete Blood Count (CBC):
    • Assess hemoglobin, hematocrit, MCV, and red cell distribution width (RDW) for classification.
  • Peripheral Blood Smear:
    • Provides clues for specific etiologies (e.g., schistocytes in hemolysis, hypersegmented neutrophils in B12 deficiency).
  • Iron Studies:
    • Evaluate serum iron, TIBC, transferrin saturation, and ferritin for iron deficiency and chronic disease.
  • Reticulocyte Count:
    • Differentiates between hypoproliferative (low reticulocyte count) and hyperproliferative anemias (high reticulocyte count).
  • Bone Marrow Biopsy:
    • Considered in cases of unexplained pancytopenia or suspected bone marrow pathology (e.g., aplastic anemia, leukemia).
Treatment Overview
  • Iron Supplementation:
    • Oral or IV iron for iron deficiency; treat underlying cause of blood loss.
  • Vitamin B12 and Folate Supplementation:
    • Necessary for macrocytic anemias due to deficiencies; address malabsorption issues if present.
  • Erythropoiesis-Stimulating Agents (ESAs):
    • Used in select cases of chronic kidney disease and anemia of chronic disease.
  • Blood Transfusion:
    • Reserved for symptomatic or severe anemia (Hb <7–8 g/dL) or acute blood loss with hemodynamic instability.
  • Managing Underlying Causes:
    • Address conditions like chronic disease, malignancies, infections, and nutritional deficiencies to prevent recurrent anemia.
Key Points
  • Anemia is classified based on MCV and pathophysiologic mechanisms, guiding diagnostic and therapeutic approaches.
  • Microcytic anemias are commonly due to iron deficiency, thalassemia, and chronic disease, with treatment based on correcting the underlying cause.
  • Normocytic anemias include hemolytic anemia, acute blood loss, and aplastic anemia, often diagnosed by reticulocyte count and peripheral smear.
  • Macrocytic anemias are frequently caused by vitamin B12 or folate deficiencies; specific testing helps confirm the deficiency type.
  • Diagnostic workup includes CBC, iron studies, reticulocyte count, and, if necessary, bone marrow biopsy.
  • Treatment is etiology-specific, with iron and vitamin supplementation for deficiencies, transfusions for severe anemia, and ESAs in select chronic conditions.

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