8.2 Anemias: Classification, Iron Deficiency, Megaloblastic, Hemolytic, and Hemoglobinopathies

Anemias: Classification, Iron Deficiency, Megaloblastic, Hemolytic, and Hemoglobinopathies

Anemia is defined clinically as a reduction below reference limits in the concentration of circulating hemoglobin, hematocrit, or total red blood cell mass, resulting in reduced oxygen delivery to tissues. Diagnostic investigation of anemia requires integrating quantitative automated red cell indices, peripheral smear cytomorphology, biochemical markers of iron kinetics and hemolysis, and hemoglobin molecular characterization. For MTLE licensure candidates, mastery of diagnostic algorithms separating microcytic, macrocytic, and hemolytic anemias is essential.

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1. Classification of Anemias: Morphologic vs Kinetic Frameworks

Two complementary systems are employed in clinical hematology to classify anemias:

                         DIAGNOSTIC ANEMIA CLASSIFICATION

                        ┌───────────────────────────────┐
                        │        ANEMIA DIAGNOSIS       │
                        │   (↓ Hb, ↓ Hct, ↓ RBC count)   │
                        └───────────────┬───────────────┘
                                        │
          ┌─────────────────────────────┴─────────────────────────────┐
          ▼                                                           ▼
  ┌───────────────────────────────┐           ┌───────────────────────────────┐
  │    KINETIC CLASSIFICATION     │           │   MORPHOLOGIC CLASSIFICATION  │
  │   (Marrow Response / RPI)     │           │       (MCV / RBC Size)        │
  └───────────────┬───────────────┘           └───────────────┬───────────────┘
                  │                                           │
        ┌─────────┴─────────┐                   ┌─────────────┼─────────────┐
        ▼                   ▼                   ▼             ▼             ▼
  ┌───────────┐       ┌───────────┐       ┌───────────┐ ┌───────────┐ ┌───────────┐
  │ Hypoprolif│       │ Hyperpro- │       │Microcytic │ │Normocytic │ │Macrocytic │
  │ (RPI < 2) │       │ liferative│       │(MCV <80fL)│ │(80-100 fL)│ │(MCV>100fL)│
  │ IDA, ACD, │       │ (RPI > 2) │       │ IDA, ACD, │ │ Acute loss│ │ Megalobl. │
  │ Aplastic  │       │ Hemolysis,│       │ Thalas,   │ │ Aplastic, │ │ Non-meg.  │
  │           │       │ Acute loss│       │ Siderobl. │ │ Hemolytic │ │ (B12, Fol)│
  └───────────┘       └───────────┘       └───────────┘ └───────────┘ └───────────┘

Kinetic Classification (Bone Marrow Response)

Assesses whether anemia results from decreased red cell production (hypoproliferation) or increased red cell destruction/loss (survival defect / hemorrhage):

1. Corrected Reticulocyte Count (CRC): $\text{CRC (\%)} = \text{Reticulocyte (\%)} \times \left( \frac{\text{Patient's Hematocrit}}{45} \right)$
2. Reticulocyte Production Index (RPI): Corrects for the premature release of "stress reticulocytes" from marrow into circulation, which prolongs peripheral blood reticulocyte maturation from 1 day up to 2.5 days: $\text{RPI} = \frac{\text{CRC}}{\text{Maturation Factor (days)}}$ (Maturation factor: Hct 45% = 1.0; Hct 35% = 1.5; Hct 25% = 2.0; Hct 15% = 2.5)
   • $\text{RPI} < 2$: Hypoproliferative anemia; ineffective erythropoiesis or impaired bone marrow response (e.g., IDA, aplastic anemia, ACD, marrow infiltration).
   • $\text{RPI} > 2$ (or $>3$): Hyperproliferative response; adequate compensatory bone marrow erythropoiesis (e.g., active hemolysis, acute blood loss after resuscitation).

Morphologic Classification (Mean Cell Volume)

• Microcytic Hypochromic ($MCV < 80\text{ fL}$, $MCHC < 32\text{ g/dL}$): Defective hemoglobin production.
• Normocytic Normochromic ($MCV\ 80\text{--}100\text{ fL}$, $MCHC\ 32\text{--}36\text{ g/dL}$): Acute hemorrhage, hemolysis, aplastic anemia, early anemia of chronic disease, renal failure.
• Macrocytic ($MCV > 100\text{ fL}$): Megaloblastic (DNA synthesis defect) vs non-megaloblastic (membrane lipid accumulation).

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2. Differential Diagnosis of Microcytic Hypochromic Anemias

Four major clinical disorders produce microcytic hypochromic red cells. Accurately distinguishing them requires analyzing serum iron, total iron-binding capacity (TIBC), transferrin saturation, serum ferritin, and erythrocyte indices.

1. Iron Deficiency Anemia (IDA)

• Pathophysiology: Most common anemia worldwide. Progresses through three distinct sequential stages:
  1. Stage 1: Iron Store Depletion: Bone marrow hemosiderin stores and serum ferritin decrease (<15 ng/mL). Serum iron, TIBC, and red cell indices remain normal. Ferritin is the most sensitive and specific single biochemical index of early depleted iron stores.
  2. Stage 2: Iron Deficient Erythropoiesis: Depleted storage iron compromises transport. Serum iron drops, TIBC rises, transferrin saturation falls below 15%, and Free Erythrocyte Protoporphyrin (FEP / Zinc Protoporphyrin ZPP) accumulates due to lack of iron for ferrochelatase insertion. Red cell morphology remains normocytic.
  3. Stage 3: Frank Iron Deficiency Anemia: Microcytic hypochromic anemia develops with decreased Hb/Hct, low MCV, low MCHC, and markedly elevated RDW (>15.0%). Smear demonstrates microcytic hypochromic cells, prominent pencil/cigar cells, and occasional target cells.

2. Anemia of Chronic Disease / Inflammation (ACD)

• Pathophysiology: Driven by chronic systemic inflammation, infections, autoimmune disorders (e.g., rheumatoid arthritis), or malignancies. Inflammatory cytokines—primarily Interleukin-6 (IL-6)—stimulate hepatocytes to synthesize and secrete hepcidin, the master iron-regulatory peptide hormone.
• Hepcidin Mechanism: Hepcidin binds directly to the basolateral iron exporter ferroportin-1 on duodenal enterocytes and reticuloendothelial macrophages, inducing its internalization, ubiquitination, and lysosomal degradation.
• Consequence: Iron absorption in the gut is blocked, and recycled iron derived from senescent RBCs is trapped inside macrophage storage pools. Erythroid precursors are starved of functional iron despite abundant total body iron stores.
• Laboratory Profile: Low serum iron, decreased or low-normal TIBC (transferrin is a negative acute-phase reactant), normal to elevated serum ferritin (ferritin is a positive acute-phase reactant), normal transferrin saturation ($15\%\text{ to }25\%$), and normal RDW.

3. Sideroblastic Anemia

• Pathophysiology: Defective synthesis of protoporphyrin IX or abnormal incorporation of iron into the heme ring. Can be hereditary (X-linked mutation in erythroid-specific $\delta$-aminolevulinate synthase, ALAS2) or acquired (lead poisoning, alcohol toxicity, myelodysplastic syndrome with ring sideroblasts, MDS-RS driven by SF3B1 mutations).
• Diagnostic Hallmark: Bone marrow examination stained with Perls' Prussian blue demonstrates ring sideroblasts—erythroblasts containing $\ge 5$ Prussian-blue-positive siderotic iron granules encircling one-third or more of the nucleus, representing iron trapped within abnormal perinuclear mitochondria.
• Laboratory Profile: Elevated serum iron, elevated transferrin saturation ($>50\%\text{ to }80\%$), markedly elevated serum ferritin, and bone marrow hemosiderin excess. Peripheral smear exhibits a dimorphic erythrocyte population (microcytic hypochromic cells coexisting with normocytic or macrocytic cells), coarse basophilic stippling, and Pappenheimer bodies.

4. Thalassemia Syndromes (Quantitative Globin Defects)

Thalassemias represent hereditary hemolytic disorders characterized by quantitative reduction or complete absence of synthesis of normal globin peptide chains (alpha or beta chains):

Beta-Thalassemia Syndromes

• Beta-Thalassemia Minor (Trait): Heterozygous reduction in $\beta$-chain synthesis ($\beta/\beta^+$ or $\beta/\beta^0$). Clinical presentation is mild, asymptomatic microcytic anemia.
  • Key Diagnostic Features: Disproportionately high RBC count ($>5.0 \times 10^{12}/\text{L}$) despite low hemoglobin ($10\text{--}12\text{ g/dL}$); very low MCV ($55\text{--}72\text{ fL}$); normal RDW ($11.5\%\text{--}14.5\%$).
  • Mentzer Index: $\frac{\text{MCV}}{\text{RBC count (millions/}\mu\text{L)}}$. A value $<13$ strongly favors thalassemia trait, whereas a value $>13$ strongly points toward iron deficiency anemia.
  • Hemoglobin Electrophoresis: Diagnostic elevation of $HbA_2 > 3.5\%$ (typically $4.0\%\text{ to }7.0\%$; normal $1.5\%\text{--}3.5\%$) and slightly elevated $HbF$ ($1\%\text{ to }3\%$).
• Beta-Thalassemia Major (Cooley's Anemia): Homozygous complete absence or extreme reduction of $\beta$-chain production ($\beta^0/\beta^0$ or $\beta^+/\beta^+$). Excess unbound free $\alpha$-chains precipitate within erythroid precursors, causing severe intramedullary apoptosis (ineffective erythropoiesis) and severe peripheral extravascular hemolysis.
  • Clinical Features: Severe transfusion-dependent microcytic anemia appearing at 6 months of age (as fetal HbF switches to adult HbA), massive hepatosplenomegaly, skeletal deformities ("hair-on-end" skull radiograph, maxillary hyperplasia), and iron overload/hemosiderosis.
  • Smear and Electrophoresis: Marked anisopoikilocytosis, abundant target cells, teardrop cells, prominent nucleated RBCs (metarubricytes), and basophilic stippling. Electrophoresis reveals $HbF$ comprising $70\%\text{ to }95\%$ of total hemoglobin, with total absence or trace amounts of HbA.

Alpha-Thalassemia Syndromes

Alpha-globin chains are encoded by four genes (two on each chromosome 16: $\alpha\alpha/\alpha\alpha$). Alpha-thalassemias typically result from gene deletions:

Microcytic Anemias Laboratory Differential Matrix

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3. Macrocytic Anemias: Megaloblastic vs Non-Megaloblastic

Macrocytic anemias ($MCV > 100\text{ fL}$) are divided into megaloblastic and non-megaloblastic etiologies based on the mechanism of cellular enlargement:

                      MACROCYTIC ANEMIA WORKUP

                         ┌───────────────────┐
                         │   MCV > 100 fL    │
                         └─────────┬─────────┘
                                   │
         ┌─────────────────────────┴─────────────────────────┐
         ▼                                                   ▼
  ┌─────────────────────────────┐     ┌─────────────────────────────┐
  │    MEGALOBLASTIC ANEMIA     │     │  NON-MEGALOBLASTIC MACROCYTE│
  │    (Impaired DNA Synth)     │     │   (Preserved DNA Synth)     │
  └──────────────┬──────────────┘     └──────────────┬──────────────┘
                 │                                   │
  • Nuclear-cytoplasmic dyssynchrony  • Round macrocytes (normal N:C)
  • Macro-ovalocytes, pancytopenia    • Normal neutrophil segmentation
  • Hypersegmented neutrophils (≥5)   • Normal LD, normal homocysteine
  • Ineffective erythropoiesis (↑↑LD) • Causes: Liver disease, alcohol,
                 │                      reticulocytosis, hypothyroidism
         ┌───────┴───────┐
         ▼               ▼
  ┌─────────────┐ ┌─────────────┐
  │ Vitamin B12 │ │   Folate    │
  │ Deficiency  │ │ Deficiency  │
  └──────┬──────┘ └──────┬──────┘
         │               │
  • ↑ Homocysteine• ↑ Homocysteine
  • ↑↑ MMA (Spec.)• Normal MMA
  • Neuro symptoms• NO neuro sx
  • Pernicious An (anti-IF / APCA)

Megaloblastic Anemia: Pathophysiology and Morphology

• Pathogenesis: Arises from deficient coenzymes required for DNA thymidine monophosphate ($dTMP$) synthesis—specifically Vitamin B12 (Cobalamin) or Folate (Vitamin B9). While nuclear DNA replication stalls, cytoplasmic RNA and protein synthesis proceed at normal rates. This produces classic nuclear-cytoplasmic dyssynchrony: immature, delicate, lacy nuclear chromatin coexisting with abundant, hemoglobinized cytoplasm.
• Peripheral Blood Smear Hallmarks:
  1. Macro-ovalocytes: Large, oval-shaped erythrocytes lacking central pallor.
  2. Neutrophil Hypersegmentation: Presence of $\ge 5\%$ of circulating neutrophils containing 5 or more distinct nuclear lobes, or the presence of a single neutrophil with 6 or more lobes. Hypersegmented neutrophils are the earliest and most reliable morphologic indicator of megaloblastic transformation.
  3. Pancytopenia: Concomitant leukopenia and thrombocytopenia with giant platelets.
  4. Ineffective Erythropoiesis: Intense intramedullary apoptosis of megaloblastic erythroblasts releases intracellular enzymes, causing profound elevations in Lactate Dehydrogenase (LD, predominantly LD-1 and LD-2), elevated indirect bilirubin, and reduced haptoglobin.

Biochemical Differentiation: Vitamin B12 vs Folate Deficiency

Both deficiencies cause megaloblastic macrocytic anemia and elevated plasma homocysteine. However, they are biochemically and clinically distinguished by Methylmalonic Acid (MMA) and neurologic status:

• Homocysteine Pathway: Both B12 (methylcobalamin) and $N^5$-methyl-THF are required to convert homocysteine to methionine via methionine synthase. Hence, total homocysteine is elevated in BOTH B12 and folate deficiencies.
• Methylmalonyl-CoA Pathway: Vitamin B12 (adenosylcobalamin) is the mandatory coenzyme for methylmalonyl-CoA mutase, converting methylmalonyl-CoA to succinyl-CoA. In B12 deficiency, methylmalonic acid cannot be metabolized, causing markedly elevated serum and urine Methylmalonic Acid (MMA). In folate deficiency, MMA remains strictly normal.
• Neurologic Consequences: Vitamin B12 deficiency leads to defective myelin sheath maintenance, causing Subacute Combined Degeneration (SCD) of the posterior and lateral columns of the spinal cord (loss of vibration and position sense, paresthesias, spastic ataxia, dementia). Neurologic symptoms occur in B12 deficiency but are absent in folate deficiency. Administering folate to a B12-deficient patient corrects the hematologic anemia but allows irreversible neurologic destruction to progress.
• Pernicious Anemia (PA): Autoimmune destruction of gastric parietal cells leading to achlorhydria and absence of Intrinsic Factor (IF). Characterized by diagnostic autoantibodies: Anti-Intrinsic Factor antibodies (Types I and II; highly specific) and Anti-Parietal Cell Antibodies (APCA; sensitive but less specific).

Non-Megaloblastic Macrocytic Anemias

Characterized by round macrocytes, normal neutrophil segmentation, normal LD, and absence of nuclear-cytoplasmic dyssynchrony. Major etiologies include:

• Chronic Liver Disease and Alcoholism: Excessive membrane surface area caused by accumulation of unesterified cholesterol and phospholipids in the outer lipid bilayer.
• Reticulocytosis: Elevated reticulocytes ($MCV\ 110\text{--}130\text{ fL}$) transiently increase the automated MCV.
• Hypothyroidism and Aplastic Anemia.

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4. Hemolytic Anemias: Intravascular vs Extravascular Hemolysis

Hemolytic anemias are characterized by accelerated premature destruction of erythrocytes (lifespan $<120\text{ days}$). Hemolysis is categorized anatomically based on the site of destruction:

                      HEMOLYSIS PATHWAY COMPARISON

         INTRAVASCULAR HEMOLYSIS              EXTRAVASCULAR HEMOLYSIS
         (RBC Lysis in Blood Vessels)         (Phagocytosis in Spleen/Liver)
                     │                                      │
                     ▼                                      ▼
             Free Hemoglobin                         Macrophage Lysis
                     │                                      │
         ┌───────────┴───────────┐                          ▼
         ▼                       ▼                 Heme Oxygenase Cleavage
  Haptoglobin Binding     Renal Filtration                  │
         │                       │                          ▼
         ▼                       ▼                  Protoporphyrin Ring
   Markedly Absent         Hemoglobinuria                   │
   Serum Haptoglobin             │                          ▼
                                 ▼                 Unconjugated Bilirubin
                          Tubular Uptake                    │
                                 │                          ▼
                                 ▼                 ↑ Plasma Indirect Bilirubin
                         Hemosiderinuria                    │
                         (Prussian Blue)                    ▼
                                                   ↑ Urine / Fecal Urobilinogen

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5. Qualitative Hemoglobinopathies: Sickle Cell Syndromes, HbC, and HbE

Hemoglobinopathies arise from point mutations altering globin polypeptide amino acid sequences, modifying hemoglobin solubility, electrical charge, or oxygen affinity.

1. Sickle Cell Anemia (Hemoglobin S)

• Molecular Genetics: Point mutation in the $\beta$-globin gene at codon 6: replacement of hydrophilic glutamic acid with hydrophobic valine ($\beta6\text{ Glu}\to\text{Val}$). Genotype: Homozygous HbSS (Sickle Cell Disease) or Heterozygous HbAS (Sickle Cell Trait).
• Pathophysiology: When deoxygenated, the nonpolar valine binds a complementary hydrophobic pocket on the adjacent $\beta$-chain, causing polymer tactoid formation, sickling, microvascular vaso-occlusion, tissue infarction, and autosplenectomy (leaving Howell-Jolly bodies).
• Laboratory Screening Tests:
  1. Sodium Metabisulfite Test: A 2% sodium metabisulfite solution deoxygenates hemoglobin on a sealed glass slide, inducing sickling visible under light microscopy within 15 to 30 minutes.
  2. Sodium Dithionite Tube Solubility Test: Erythrocytes are lysed in a concentrated phosphate buffer solution containing sodium dithionite (reducing agent) and saponin. Deoxygenated HbS is insoluble in high-molarity phosphate buffer, forming a cloudy, turbid suspension that obscures black reader lines on a background chart. Soluble hemoglobins (HbA, HbF) leave a clear, transparent solution.
     • Limitations: Does NOT differentiate HbSS disease from HbAS trait; false negatives occur in infants $<6\text{ months}$ (predominant HbF) or patients with severe anemia ($Hb < 7.0\text{ g/dL}$); false positives occur in multiple myeloma or extreme hyperlipidemia.

2. Hemoglobin C (HbC)

• Molecular Genetics: Point mutation at codon 6 of the $\beta$-globin gene substituting positively charged lysine for glutamic acid ($\beta6\text{ Glu}\to\text{Lys}$).
• Morphology: Intracellular dehydration produces numerous target cells and pathognomonic HbC crystals—dense, dark, hexagonal, oblong, or "bar-of-gold" crystals within the cell cytoplasm, particularly evident in post-splenectomy patients.

3. Hemoglobin E (HbE)

• Molecular Genetics: Point mutation at codon 26 of the $\beta$-globin gene substituting lysine for glutamic acid ($\beta26\text{ Glu}\to\text{Lys}$).
• Epidemiology: Extremely prevalent in mainland Southeast Asia (Thailand, Cambodia, Laos, Myanmar). It is less common in Filipinos, among whom alpha-thalassemia deletions (including the --SEA and --FIL types) are the leading inherited hemoglobin disorder. HbE causes mild microcytic hypochromic red cells with prominent target cells. When co-inherited with $\beta$-thalassemia (HbE/$\beta$-thalassemia), it manifests as a severe thalassemia major phenotype.

4. Dual-pH Hemoglobin Electrophoresis

Definitive clinical identification of hemoglobinopathies relies on combining Alkaline Cellulose Acetate Electrophoresis and Acid Citrate Agar Electrophoresis:

                    HEMOGLOBIN ELECTROPHORESIS PATTERNS

  ALKALINE ELECTROPHORESIS (pH 8.4 - 8.6, Cellulose Acetate)
  Cathode (-) ──────────────────────────────────────────────> Anode (+)
                 C                S            F          A
              (HbC, A2,        (HbS, D,      (HbF)      (HbA)
               HbE, O)           HbG)
              "Crawls"         "Slow"        "Fast"    "Accelerates"

  ACID ELECTROPHORESIS (pH 6.0 - 6.2, Citrate Agar)
  Cathode (-) ──────────────────────────────────────────────> Anode (+)
                 F                A            S          C
               (HbF)           (HbA, D,      (HbS)      (HbC)
                               HbG, E, O)
              "Fast"         "Accelerates"   "Slow"    "Crawls"

• Alkaline Electrophoresis (pH 8.4 to 8.6): At alkaline pH, hemoglobin molecules carry net negative charges and migrate toward the anode (+). Net charge differences drive migration: $\text{Cathode } (-) \longrightarrow \mathbf{C} \longrightarrow \mathbf{S} \longrightarrow \mathbf{F} \longrightarrow \mathbf{A} \longrightarrow \text{Anode } (+)$
  • Mnemonic: Crawls, Slow, Fast, Accelerates.
  • Comigrations at Alkaline pH: HbA2, HbC, HbE, and HbO-Arab co-migrate to the "C" position; HbS, HbD-Los Angeles, and HbG-Philadelphia co-migrate to the "S" position.
• Acid Citrate Agar Electrophoresis (pH 6.0 to 6.2): Essential secondary confirmation. Citrate agar separation depends on the specific stereochemical affinity of hemoglobin variants for agaropectin in the agar gel rather than net electrical charge: $\text{Cathode } (-) \longrightarrow \mathbf{F} \longrightarrow \mathbf{A} \longrightarrow \mathbf{S} \longrightarrow \mathbf{C} \longrightarrow \text{Anode } (+)$
  • Resolution: HbS separates cleanly from HbD and HbG (which migrate with HbA); HbC separates cleanly from HbE and HbO-Arab (which migrate with HbA).