3.1 Anemias: Microcytic, Normocytic & Macrocytic Etiologies

Diagnostic Framework for Anemia

Evaluation of anemia on USMLE Step 2 CK begins with the Mean Corpuscular Volume (MCV) to categorize red blood cell (RBC) size into microcytic (< 80 fL), normocytic (80–100 fL), or macrocytic (> 100 fL). Once MCV is established, clinical history, peripheral blood smear, reticulocyte response, and targeted biochemical assays narrow the differential diagnosis.

To evaluate bone marrow response, the raw reticulocyte percentage must be converted to the Reticulocyte Index (RI) (also called Corrected Reticulocyte Count):$\text{RI} = \text{Reticulocyte } \% \times \left( \frac{\text{Patient Hct}}{\text{Normal Hct (45\%)}} \right) \div \text{Shift Correction Factor}$

A shift correction factor of 2.0 is applied if polychromasia (shift reticulocytes) is present or hematocrit is severely depressed (< 25%). An RI > 2% indicates robust bone marrow erythrocyte production in response to peripheral RBC destruction (hemolysis) or acute blood loss. An RI < 2% indicates an inadequate marrow response, pointing to hypoproliferative bone marrow failure, nutrient deficiency, or systemic suppression.

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Microcytic Anemias (MCV < 80 fL)

Microcytic anemias stem from defective hemoglobin synthesis, involving either heme production (iron deficiency, sideroblastic anemia, lead toxicity, chronic disease) or globin chain synthesis (thalassemias).

Etiologies & Iron Panel Differentiation

1. Iron Deficiency Anemia (IDA): The most common cause of anemia worldwide. Caused by chronic blood loss (gastrointestinal malignancy in older adults, menorrhagia in premenopausal women) or malabsorption (celiac disease, post-gastrectomy). Characterized by serum ferritin < 30 ng/mL, high Total Iron Binding Capacity (TIBC > 400 µg/dL), low transferrin saturation (< 15%), and an elevated Red Cell Distribution Width (RDW > 15%) reflecting anisocytosis.
2. Anemia of Chronic Disease / Inflammation (ACD/AI): Driven by inflammatory cytokines (IL-6, TNF-alpha) that stimulate hepatic production of hepcidin. Hepcidin degrades ferroportin on enterocytes and macrophages, trapping iron within storage sites. Characterized by elevated serum ferritin (> 300 ng/mL), low TIBC (< 250 µg/dL), low serum iron, and low transferrin saturation (15–20%).
3. Thalassemias: Quantitative globin chain synthesis defects. Beta-thalassemia minor (heterozygous beta-gene mutation) presents with mild microcytic anemia (MCV often < 70 fL despite Hgb 10–11 g/dL), normal or elevated serum ferritin, normal RDW, and characteristic HbA2 > 3.5% (and elevated HbF) on hemoglobin electrophoresis. Alpha-thalassemia trait (two-gene deletion: --/alpha alpha or -alpha/-alpha) presents with microcytosis and normal hemoglobin electrophoresis in adults.
4. Sideroblastic Anemia: Defective heme synthesis leading to iron accumulation in erythrocyte mitochondria. Primary cause is X-linked ALAS2 mutation; acquired causes include alcohol toxicity, lead poisoning, and isoniazid (B6 deficiency). Diagnosed by ringed sideroblasts (≥ 15% on Prussian blue bone marrow stain), high serum iron, high ferritin, and transferrin saturation > 50%.
5. Lead Toxicity: Inhibits delta-aminolevulinic acid dehydratase (ALAD) and ferrochelatase. Presents with abdominal colic, peripheral neuropathy (foot/wrist drop), basophilic stippling on blood smear, and elevated blood lead level (BLL ≥ 3.5 µg/dL in children, ≥ 5 µg/dL in adults).

Diagnostic Algorithm: Microcytic Anemia

[ Patient with Microcytic Anemia (MCV < 80 fL) ]
                       │
          Measure Serum Ferritin & Iron Panel
                       │
        ┌──────────────┼──────────────┐
        ▼              ▼              ▼
Ferritin < 30 ng/mL  Ferritin > 300 ng/mL  Normal Ferritin / TIBC
        │              │              │
        ▼              ▼              ▼
 Iron Deficiency   Anemia of Chronic   Hemoglobin Electrophoresis
  (Check GI/GU       Disease (ACD)    ┌───────┴───────┐
  bleeding sites)    (Hepcidin-driven)▼               ▼
                                   HbA2 > 3.5%    Normal HbA2/HbF
                                      │               │
                                      ▼               ▼
                                  Beta-Thal      Alpha-Thal Trait
                                    Minor        or Lead/B6 evaluation

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Normocytic Anemias (MCV 80–100 fL)

Normocytic anemias require immediate evaluation of the Reticulocyte Index (RI) to branch into hyperproliferative (RI > 2%) vs hypoproliferative (RI < 2%) categories.

Hyperproliferative (RI > 2%): Hemolytic Anemias & Hemorrhage

• Intrinsic Hemolytic Anemias:
  • Hereditary Spherocytosis: Autosomal dominant defect in spectrin/ankyrin. Shows spherocytes on smear, positive osmotic fragility test, negative Direct Antiglobulin Test (DAT/Coombs), and elevated MCHC (> 36 g/dL). Splenectomy is curative for severe anemia.
  • G6PD Deficiency: X-linked recessive enzymopathy predisposing RBCs to oxidative stress (fava beans, sulfa drugs, infections). Smear shows Heinz bodies (crystal violet stain) and bite cells.
  • Sickle Cell Disease (HbSS): Glutamate to valine mutation at codon 6 of beta-globin chain. Presents with vaso-occlusive crises, acute chest syndrome (fever, chest pain, new infiltrate), and splenic autoinfarction by adulthood.
  • Paroxysmal Nocturnal Hemoglobinuria (PNH): Acquired PIGA gene mutation causing loss of GPI-anchored proteins (CD55 and CD59). Presents with nocturnal hemoglobinuria, severe thrombosis (hepatic vein / Budd-Chiari syndrome), and positive flow cytometry for CD55/CD59 deficiency.
• Extrinsic Hemolytic Anemias:
  • Autoimmune Hemolytic Anemia (AIHA):
    • Warm AIHA (IgG): RBC destruction at 37°C in spleen. Associated with CLL, SLE, and drugs (penicillin, alpha-methyldopa). Positive IgG Direct Coombs test. Treated with corticosteroids.
    • Cold AIHA (IgM): Complement-mediated RBC agglutination at < 37°C in acral areas. Associated with Mycoplasma pneumoniae and EBV. Positive C3 Direct Coombs test. Treated with cold avoidance; corticosteroids are ineffective.

Hypoproliferative (RI < 2%): Marrow Failure

• Aplastic Anemia: Pancytopenia (Hgb < 10 g/dL, Platelets < 50,000/µL, ANC < 1,500/µL) with hypocellular marrow (> 70% fat spaces). Caused by drugs (carbamazepine, NSAIDs), viruses (parvovirus B19, EBV, hepatitis), or autoimmune destruction. Treated with bone marrow transplant or immunosuppression (cyclosporine + antithymocyte globulin).
• Chronic Kidney Disease (CKD): Decreased erythropoietin (EPO) synthesis by renal peritubular interstitial cells when eGFR drops < 30 mL/min/1.73 m². Treated with recombinant human EPO or darbepoetin after ensuring adequate iron stores.

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Macrocytic Anemias (MCV > 100 fL)

Macrocytic anemias are subdivided based on peripheral blood smear evidence of megaloblastic maturation (hypersegmented neutrophils: ≥ 5 lobes in > 5% of PMNs or 1 PMN with ≥ 6 lobes).

Megaloblastic vs Non-Megaloblastic Etiologies

1. Megaloblastic Anemia (Impaired DNA Synthesis):
   • Vitamin B12 (Cobalamin) Deficiency: Caused by pernicious anemia (anti-intrinsic factor / anti-parietal cell antibodies), ileal resection, or strict vegan diet. Clinical findings include macrocytic anemia, glossitis, and subacute combined degeneration (SCD) of the spinal cord (posterior columns: loss of vibration/proprioception; lateral corticospinal tracts: spastic paraparesis). Lab testing reveals elevated serum homocysteine AND elevated methylmalonic acid (MMA > 0.4 µmol/L).
   • Folate Deficiency: Caused by alcoholism, malnutrition, pregnancy, or antifolate drugs (methotrexate, trimethoprim, phenytoin). Clinical findings mirror B12 deficiency but lack neurologic deficits. Lab testing reveals elevated serum homocysteine with NORMAL methylmalonic acid.
2. Non-Megaloblastic Macrocytic Anemia (Normal DNA Synthesis):
   • Causes include chronic alcoholism (direct marrow toxicity), liver disease (lipid deposition on RBC membranes), hypothyroidism, and drugs such as hydroxyurea or AZT.