Hematology Chapter 21: Intrinsic Defects Leading to Increased RBC Destruction

What is hereditary spherocytosis? What are three “textbook” clinical findings?

Hereditary Spherocytosis (HS) is an inherited intrinsic hemolytic anemia caused by defects in RBC membrane proteins such as:

• Spectrin

• Ankyrin

• Band 3

• Protein 4.2

These defects disrupt vertical membrane interactions, causing membrane loss and formation of spherocytes that become trapped and destroyed in the spleen.

Three textbook clinical findings:

1. Splenomegaly

2. Jaundice

3. Gallstones (pigment stones)

Other common findings:

• Hemolytic anemia

• Reticulocytosis

• Spherocytes on smear

• Family history of anemia

What CBC results are seen with hereditary spherocytosis?

Characteristic CBC Findings

✅ Decreased hemoglobin

✅ Increased reticulocyte count

✅ Increased RDW

✅ Increased MCHC (hallmark finding)

Peripheral Smear Findings

✅ Spherocytes

✅ Polychromasia

✅ Anisocytosis

Additional Laboratory Findings

✅ Increased indirect bilirubin

✅ Increased LDH

✅ Decreased haptoglobin

✅ Negative DAT (helps distinguish HS from autoimmune hemolytic anemia)

What is PNH?

Paroxysmal Nocturnal Hemoglobinuria (PNH) is an acquired stem cell disorder caused by a mutation in the PIGA gene.

The mutation causes loss of GPI-linked proteins:

• CD55 (DAF)

• CD59

Without CD55 and CD59, RBCs become susceptible to complement-mediated intravascular hemolysis.

Classic Features Triad

1. Hemolytic anemia

2. Hemoglobinuria (especially dark morning urine)

3. Thrombosis

Confirmatory Test

✅ Flow cytometry for CD55 and CD59 deficiency

Treatment

✅ Eculizumab

✅ Supportive care

What is the most common RBC enzyme deficiency worldwide?

Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency

G6PD deficiency is the most common RBC enzyme deficiency in the world.

Function of G6PD

Produces NADPH through the:

Hexose Monophosphate (HMP) Shunt

NADPH protects RBCs against oxidative damage.

Triggering Factors

• Sulfa drugs

• Primaquine

• Infections

• Fava beans

Morphology

✅ Heinz bodies

✅ Bite cells

What is microangiopathic hemolytic anemia (MAHA)? List two causes/mechanisms.

Definition

Microangiopathic hemolytic anemia (MAHA) occurs when RBCs are physically fragmented while passing through damaged small blood vessels.

Morphology

✅ Schistocytes

✅ Fragmented RBCs

Causes (know at least two)

1. Disseminated Intravascular Coagulation (DIC)

2. Thrombotic Thrombocytopenic Purpura (TTP)

Additional causes:

• Hemolytic Uremic Syndrome (HUS)

• Prosthetic heart valves

• Severe hypertension

Mechanism

Mechanical shearing of RBCs

↓

Fragmentation hemolysis

↓

Schistocyte formation

What are the two most common enzyme abnormalities of the RBC ? Tell us a little about each.

A. G6PD Deficiency Defect

Deficiency of:

Glucose-6-Phosphate Dehydrogenase

Pathway Affected

Hexose Monophosphate (HMP) Shunt

Consequence

↓ NADPH

↓

↓ Reduced glutathione (GSH)

↓

Oxidative damage

↓

Hemolysis

Smear Findings

✅ Heinz bodies

✅ Bite cells

Type of Hemolysis

Usually acute episodic hemolysis after oxidative stress.

---

B. Pyruvate Kinase Deficiency Defect

Deficiency of:

Pyruvate Kinase

Pathway Affected

Embden-Meyerhof Pathway (glycolysis)

Consequence

↓ ATP production

↓

Membrane failure

↓

Chronic hemolysis

Smear Findings

✅ Burr cells (echinocytes)

Clinical Presentation

✅ Chronic nonspherocytic hemolytic anemia (CNSHA)

Which intrinsic RBC property is most important for allowing RBCs to pass through splenic sinusoids?

Deformability

Osmolality

Hemoglobin F

Agglutination

Deformability

RBC deformability allows cells to change shape and pass through narrow splenic cords and capillaries.

Hereditary spherocytosis is most commonly caused by defects involving which membrane interaction?

Vertical interactions

Horizontal interactions

Globin synthesis

Heme synthesis

Vertical interactions

HS results from defective vertical interactions between the membrane skeleton and lipid bilayer.

Which RBC morphology is characteristic of hereditary spherocytosis?

Elliptocytes

Target cells

Spherocytes

Schistocytes

Spherocytes

Loss of membrane surface area produces dense spherical RBCs.

Which CBC parameter is classically increased in hereditary spherocytosis?

MCV

MCHC

Platelet count

MPV

MCHC

The classic triad of hereditary spherocytosis includes anemia, jaundice, and what additional finding?

Lymphadenopathy

Splenomegaly

Leukocytosis

Neutropenia

Splenomegaly

Hereditary elliptocytosis is most commonly caused by defects in which type of membrane interaction?

Vertical

Horizontal

Complement-mediated

Oxidative

Horizontal

HE results from abnormal horizontal interactions involving spectrin and related proteins.

Which RBC shape is most characteristic of hereditary elliptocytosis?

Spherocytes

Elliptocytes

Stomatocytes

Acanthocytes

Elliptocytes

Which test is considered the preferred screening test for hereditary spherocytosis?

DAT

Eosin-5-maleimide binding test

PTT

G6PD screen

Eosin-5-maleimide binding test

Which test demonstrates increased fragility of spherocytes in hypotonic saline?

DAT

Osmotic fragility

PT

Flow cytometry

Osmotic fragility

Spherocytes lyse more readily in hypotonic solutions.

Stomatocytes characteristically show what central pallor appearance?Q

Bull's-eye

Mouth-shaped slit

No pallor

Fragmented center

Mouth-shaped slit

Acanthocytes are best described as RBCs with:Question 11 of 20

Uniform projections

Irregular spicules

No central pallor

Fragmented edges

Irregular spicules

Acanthocytes have irregularly spaced thorny projections.

Paroxysmal nocturnal hemoglobinuria results from a mutation in which gene?

HBB

PIGA

ANK1

G6PD

PIGA

PIGA mutations prevent formation of GPI anchors.

Which protective proteins are deficient on RBCs in PNH?Question 13 of 20

CD3 and CD4

CD55 and CD58

CD19 and CD20

CD13 and CD33

CD55 and CD59

What is the gold-standard diagnostic test for PNH?

Osmotic fragility

Flow cytometry

DAT

Ham test

Flow cytometry

Flow cytometry identifies absence of GPI-linked proteins.

What is the most common RBC enzyme deficiency worldwide?Question 15 of 20

Pyruvate kinase deficiency

Hexokinase deficiency

G6PD deficiency

ADA deficiency

G6PD deficiency

Which metabolic pathway is defective in G6PD deficiency?

Embden-Meyerhof pathway

HMP shunt

TCA cycle

Purine salvage

HMP shunt

G6PD generates NADPH through the HMP shunt.

Which inclusions are best demonstrated with supravital stain in G6PD deficiency?

Howell-Jolly bodies

Pappenheimer bodies

Heinz bodies

Cabot rings

Heinz bodies

Oxidized denatured hemoglobin forms Heinz bodies.

Which blood film finding results when splenic macrophages remove Heinz bodies?

Target cells

Bite cells

Spherocytes

Acanthocytes

Bite cells

Bite cells form after pitting of Heinz bodies.

Pyruvate kinase deficiency primarily causes decreased production of

NADPH

ATP

Iron

Bilirubin

ATP

PK deficiency limits ATP production from glycolysis.

Which RBC morphology is commonly associated with pyruvate kinase deficiency?

Echinocytes (burr cells)

Sickle cells

Target cells

Stomatocytes

Echinocytes (burr cells)

ATP depletion contributes to echinocyte formation.