Clinical Hematology: Analyzers, Hemolytic Anemias, Hemoglobinopathies, and Thalassemias

Comprehensive practice flashcards covering automated hematology analyzer principles, intrinsic and extrinsic hemolytic anemias, membrane/enzyme defects, hemoglobinopathies, thalassemias, and clinical case studies.

How do automated hematology analyzers measure red blood cell (RBC) and platelet count and volume using the Coulter principle?

A diluted sample is directed into a sheath flow and passed single-file through a measurement aperture. As each cell passes, it causes electrical resistance (impedance) proportional to its volume, which is recorded as an electrical pulse.

How is hematocrit directly measured on impedance-based automated hematology analyzers?

It is directly measured via cumulative pulse height (the sum of the electrical pulse heights produced by the RBCs passing through the aperture) and reported as a percentage.

What is the principle of the SLS-hemoglobin detection method?

Sodium lauryl sulphate (SLS) is a cyanide-free reagent that lyses RBCs and WBCs, alters globin, and oxidizes the heme group. Hydrophilic groups of SLS bind the heme to form a stable SLS-HGB complex whose monochromatic light absorbance is measured photometrically and is proportional to hemoglobin concentration in $\text{g/dL}$.

In automated flow cytometry, what cellular characteristics do Forward Scatter (FSC), Side Scatter (SSC), and Side Fluorescence Light (SFL) measure?

Forward Scatter (FSC) detects cell size; Side Scatter (SSC) detects internal cell structure/complexity; Side Fluorescence Light (SFL) detects nucleic acid (RNA/DNA) content.

How does the WNR channel of a hematology analyzer differentiate nucleated RBCs (NRBCs) from white blood cells (WBCs)?

A detergent lyses RBC membranes (leaving NRBC nuclei intact) and perforates WBC membranes. A fluorescent nucleic acid dye weakly stains the exposed NRBC nuclei (due to lost cytoplasm) while intact WBCs retain abundant nucleic acid and stain much brighter, separating into two populations by fluorescence (SFL) versus cell size (FSC).

How are basophils specifically quantified in the WNR channel?

An acidic reagent lyses RBCs and strips non-basophil WBCs down to bare nuclei, whereas basophils resist lysis and remain intact. The analyzer separates basophils using forward and side scatter signals.

What five leukocyte populations are quantified by the WDF channel scattergram?

Neutrophils, lymphocytes, monocytes, eosinophils, and immature granulocytes (IG). Basophils are quantified in the WNR channel and incorporated into the final differential via an algorithm.

What is the difference between RDW-CV and RDW-SD on an RBC volume histogram?

RDW-CV is the histogram width at $1\,\text{SD}$ frequency divided by MCV $\times 100$ (elevated when $>16\%$), whereas RDW-SD is the direct histogram width measured at the $20\%$ frequency height (elevated when $>46\,\text{fL}$) and is considered a superior metric.

What is the diagnostic significance of an Immature Reticulocyte Fraction (IRF) $>10\%$ or an Immature Platelet Fraction (IPF) $>7.5\%$?

Values above these cutoffs indicate that the bone marrow is actively compensating by accelerating the production and release of young reticulocytes or platelets into circulation.

What clinical and laboratory triad distinguishes extravascular hemolysis from intravascular hemolysis?

Extravascular hemolysis occurs in macrophages of the spleen and liver, producing spherocytes, markedly elevated indirect bilirubin, and urine urobilinogen with normal haptoglobin and absent hemoglobinuria. Intravascular hemolysis occurs inside blood vessels, producing schistocytes, depleted serum haptoglobin, hemoglobinemia, and hemoglobinuria.

What is the biochemical fate of protoporphyrin released during extravascular RBC destruction?

Protoporphyrin is converted by macrophages into unconjugated (indirect) bilirubin, bound to albumin, transported to the liver, conjugated by UDP-glucuronyl transferase to glucuronic acid, and excreted in bile. Intestinal bacteria then convert conjugated bilirubin to urobilinogen, which is excreted in feces and urine.

What is the molecular pathogenesis of Hereditary Spherocytosis (HS)?

HS is an autosomal dominant intrinsic membrane defect caused by mutations in alpha-spectrin, beta-spectrin, ankyrin, or protein $4.2$. Uncoupling of the vertical skeletal lattice from the lipid bilayer causes membrane shedding, decreased surface-area-to-volume ratio, decreased deformability, and splenic clearance.

What are the typical CBC findings in Hereditary Spherocytosis?

Hemoglobin between $5\text{--}12\,\text{g/dL}$, normal MCV (around $85\,\text{fL}$), elevated MCHC ($36\text{--}38\%$ due to membrane dehydration and loss), reticulocytosis ($>3\%$), and elevated RDW ($>14.5\%$).

What is the principle of the osmotic fragility test, and why do spherocytes lyse at higher NaCl concentrations than normal RBCs?

RBCs are incubated in decreasing concentrations of hypotonic NaCl solutions for $24\,\text{hours}$. Because spherocytes have a decreased surface-area-to-volume ratio, they cannot expand when taking up water and lyse at higher NaCl concentrations than normal biconcave erythrocytes.

Why does a splenectomy alleviate anemia in Hereditary Spherocytosis?

It removes the primary extravascular filter where rigid, non-deformable spherocytes are trapped and destroyed by macrophages, prolonging RBC lifespan and preventing bilirubin gallstones, though it leaves spherocytes intact in circulation and increases infection risk.

What is the molecular defect and peripheral blood finding in Hereditary Elliptocytosis (HE)?

HE is caused by horizontal cytoskeletal defects resulting from mutations in alpha-spectrin, beta-spectrin, or protein $4.1$. Biconcave cells are produced normally but fail to snap back into shape after passing through capillary shear stress, resulting in $\ge 25\%$ elliptocytes on the blood smear.

What genetic defect causes Southeast Asian Ovalocytosis (SAO), and what unique cell morphology is observed on the peripheral smear?

An autosomal dominant $27\text{-bp}$ ($9\text{ amino acid}$) deletion in the $3'$ end of the SLC4A1 gene (encoding Band 3). It causes extremely rigid, dehydrated erythrocytes and distinctive ovalocytes with transverse bars, known as 'theta cells'.

What is the metabolic defect in Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency?

An X-linked recessive defect in the hexose-monophosphate shunt preventing adequate regeneration of NADPH, which is required by glutathione reductase to maintain reduced glutathione (GSH). Low GSH leaves erythrocytes vulnerable to oxidative stress.

What are the common clinical triggers of an acute hemolytic episode in G6PD deficiency?

Infections, oxidant drugs (such as antimalarials), and ingestion of fava beans (favism), all of which generate excessive reactive oxygen species that oxidize hemoglobin.

What are Heinz bodies, how are they visualized, and what cell type is produced when the spleen pits them out?

Heinz bodies are precipitates of denatured, aggregated hemoglobin that can only be seen with supravital stains like Brilliant Cresyl Blue or Methyl Violet (not Wright stain). When splenic macrophages remove them, they form bite cells (degmacytes) and blister cells.

What is the inheritance pattern, pathogenesis, and classic poikilocyte seen in Pyruvate Kinase (PK) deficiency?

PK deficiency is an autosomal recessive Embden-Meyerhof glycolytic pathway defect that causes insufficient ATP generation, loss of ion pumping, dehydration, membrane rigidity, and extravascular destruction. The classic poikilocytes are echinocytes (burr cells).

What defines a Microangiopathic Hemolytic Anemia (MAHA) and what are its hallmark blood smear findings?

A group of disorders where RBCs fragment as they pass through intravascular fibrin (clot) strands in small blood vessels under high shear stress. The smear features schistocytes, microspherocytes, and polychromasia.

What is the clinical triad and etiology of classic Hemolytic Uremic Syndrome (HUS)?

HUS typically affects children under $5\,\text{years old}$ following gastroenteritis caused by Shiga toxin-producing Escherichia coli (O157:H7) or Shigella dysenteriae. It presents with the triad of microangiopathic hemolytic anemia with red cell fragmentation, thrombocytopenia, and acute renal failure.

What is the underlying molecular defect in Thrombotic Thrombocytopenic Purpura (TTP)?

An autoantibody against ADAMTS13, a serum metalloprotease that cleaves ultra-large von Willebrand Factor (vWF) multimers. Without ADAMTS13 activity ($<5\%$ of normal), ultra-large vWF persists and spontaneously binds platelets, creating microthrombi.

How do coagulation test profiles differ between DIC, TTP, and HUS?

In DIC, coagulation factors and platelets are consumed systemically, causing prolonged PT, prolonged PTT, decreased fibrinogen, and markedly elevated D-dimer. In TTP and HUS, the coagulation cascade is not consumed: PT and PTT remain normal to slightly abnormal, and D-dimers are absent in HUS and only slightly elevated in $80\%$ of TTP cases.

Why is platelet transfusion contraindicated in patients with TTP?

Platelet transfusions provide additional fuel for the ultra-large vWF multimers, exacerbating widespread platelet microthrombi formation and worsening organ ischemia and mortality.

How does severe thermal injury (3rd-degree burns) cause acute hemolytic anemia?

Direct heat damages the red cell skeletal protein spectrin in circulating dermal capillaries, causing immediate loss of membrane elasticity, membrane budding, microspherocytes, schistocytes, and acute intravascular hemolysis.

What intraerythrocytic parasite transmitted by Ixodes ticks can cause acute hemolytic anemia in the United States?

Babesia microti, which replicates within erythrocytes, forms ring forms or pathognomonic tetrads ('Maltese cross'), and triggers RBC lysis and splenic macrophage clearance.

What is the genetic and molecular basis of Paroxysmal Nocturnal Hemoglobinuria (PNH)?

An acquired somatic mutation in the PIG-A gene in a CD34+ hematopoietic stem cell clone prevents the biosynthesis of the glycosyl-phosphatidylinositol (GPI) anchor, leading to an absence of complement inhibitors CD55 (decay-accelerating factor) and CD59 (membrane attack complex-inhibitory protein) on blood cells.

Why is flow cytometry of granulocytes using FLAER and CD157 preferred over RBC testing with CD59 for confirming PNH?

Complement-deficient RBCs (CD55-/CD59-) lyse and are cleared rapidly in circulation, causing false-negative or falsely low clone sizes on RBC analysis. Granulocytes survive longer and have alternative surface defenses (transmembrane CD46), allowing accurate measurement of the GPI-deficient clone via FLAER and CD157.

How do warm and cold immune hemolytic anemias differ in their predominant antibody class, hemolysis location, and DAT results?

Warm autoimmune hemolytic anemia (WAIHA) is caused by IgG antibodies reacting at $37^\circ\text{C}$, causing extravascular destruction in the spleen (spherocytes on smear; DAT positive for IgG). Cold autoimmune hemolytic anemia is caused by IgM antibodies reacting at $<30^\circ\text{C}$ that fix complement, leading to intravascular or C3b-mediated extravascular lysis (DAT positive for C3d).

What is the specific amino acid mutation and position that causes Hemoglobin S?

A point mutation in the HBB gene on chromosome 11 replacing glutamic acid with valine at the 7th position of the beta globin chain (traditionally designated codon 6).

What is the time frame for deoxy-HbS polymerization, and why does sickling not occur on every capillary transit?

Polymerization of deoxy-HbS takes approximately $2\text{--}4\,\text{minutes}$. Because normal red cells typically pass through capillary beds and return to the lungs to become reoxygenated in less time than that, most cells do not sickle on a given circulatory pass.

What is the chemical principle of the Sickledex (solubility) screening test for Sickle Cell?

Saponin lyses erythrocytes to release hemoglobin, and sodium dithionite reduces (deoxygenates) it. Deoxy-HbS is insoluble in this concentrated phosphate buffer and precipitates, causing turbidity (cloudy solution = positive; clear = negative).

Why is the Sickledex solubility test unsuitable for newborn screening?

High levels of Hemoglobin F (HbF) present at birth dilute and interfere with the polymerization of HbS, leading to false-negative solubility results.

On alkaline hemoglobin electrophoresis (pH 9.0), what is the relative migration order of hemoglobins from the cathode (-) toward the anode (+)?

At pH $9.0$, all hemoglobin variants carry a net negative charge and migrate toward the anode (+) in the order: Origin / Cathode $(-) \rightarrow \text{HbC} \rightarrow \text{HbS} \rightarrow \text{HbF} \rightarrow \text{HbA} \rightarrow$ Anode $(+)$.

What is the specific mutation, characteristic morphology, and typical CBC profile in Hemoglobin C (HbC) disease?

A point mutation in the HBB gene replacing glutamic acid with lysine at the 7th position of the beta chain. It causes intracellular hexagonal HbC crystals and target cells (codocytes), with a normal MCV and an elevated MCHC (up to $38\%$) due to cellular dehydration.

What distinct erythrocyte shapes appear on a peripheral blood smear in Hemoglobin SC disease?

Target cells (codocytes), 'pita bread' sickle cells (curved/folded cells with blunt ends), and dense, rounded 'billiard ball'-like crystalline inclusions.

What distinguishes the primary defect in thalassemias from that in hemoglobinopathies?

Hemoglobinopathies are qualitative globin defects (structural gene mutations creating abnormal globin chains), whereas thalassemias are quantitative globin defects (mutations causing decreased or absent synthesis of structurally normal globin chains).

How does the Mentzer Index differentiate beta thalassemia minor from Iron Deficiency Anemia (IDA)?

Calculated as $\text{MCV (fL)} / \text{RBC count (millions/}\mu\text{L)}$. A ratio $<13$ strongly suggests thalassemia trait (due to microcytosis with an inappropriately elevated RBC count), whereas a ratio $>13$ indicates Iron Deficiency Anemia.

What are the four clinical classifications of alpha thalassemia based on the number of deleted alpha globin genes?

Silent carrier (1 gene deleted, asymptomatic, MCV $78\text{--}80\,\text{fL}$); Alpha thalassemia minor (2 genes deleted, mild microcytic hypochromic anemia, asymptomatic); Hemoglobin H disease (3 genes deleted, chronic hemolytic anemia, splenomegaly); Alpha thalassemia major / Hydrops Fetalis (all 4 genes deleted, incompatible with life).

What are the subunit compositions of Hemoglobin Bart's and Hemoglobin H, and when are they detected?

Hemoglobin Bart's is a tetramer of excess gamma chains ($\gamma_4$) formed at birth/in utero. Hemoglobin H is a tetramer of excess beta chains ($\beta_4$) that replaces Bart's as gamma globin synthesis switches to beta globin synthesis after birth.

What is the characteristic hemoglobin electrophoresis/HPLC finding in Beta Thalassemia Minor (Trait)?

An elevated Hemoglobin A2 ($\text{HbA}_2 >3.5\%$, typically $4\text{--}6\%$), slightly elevated HbF ($1\text{--}5\%$), and a slight decrease in HbA ($90\text{--}95\%$).

Why do patients with Beta Thalassemia Intermedia and Major develop severe iron overload even in the absence of transfusions?

Severe ineffective erythropoiesis triggers massive bone marrow expansion and causes the secretion of factors (such as GDF15) that suppress hepcidin production in the liver, leading to unregulated and increased iron absorption from the duodenum and increased macrophage iron release.

Why do children with Beta Thalassemia Major develop severe bone deformities (such as 'mongoloid facies' and cortical thinning)?

Extreme ineffective erythropoiesis and anemia stimulate massive erythropoietin (EPO) secretion, causing dramatic bone marrow expansion that thins the cortical bone and leads to skeletal deformities and pathologic fractures.

In Case Study #1, a 5-year-old child presents with severe anemia (Hb $5.8\,\text{g/dL}$, Hct $19\%$), splenomegaly, RDW $26.7\%$, and sickle cells with target cells on the smear. What is the diagnosis?

Sickle Cell Anemia (HbSS / Sickle Cell Disease).

In Case Study #3, a healthy 26-year-old male has an RBC count of $6.22 \times 10^6/\mu\text{L}$, Hb $12.1\,\text{g/dL}$, MCV $61.6\,\text{fL}$, basophilic stippling, and an $\text{HbA}_2$ of $5.6\%$. What is the diagnosis?

Beta Thalassemia Minor (Beta Thalassemia Trait).

In Case Study #4, a 3-year-old with bloody diarrhea from Shiga toxin-producing E. coli develops Hb $7.0\,\text{g/dL}$, platelets $40{,}000/\mu\text{L}$, BUN $50\,\text{mg/dL}$, creatinine $4.6\,\text{mg/dL}$, and schistocytes. What is the diagnosis?

Hemolytic Uremic Syndrome (HUS).

In Case Study #7, a 56-year-old man treated with chloroquine develops an acute drop in hemoglobin ($11.4 \rightarrow 8.9\,\text{g/dL}$), marked LDH elevation ($2421\,\text{U/L}$), and bite cells on blood smear. What is the diagnosis?

Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency experiencing an acute hemolytic crisis precipitated by an oxidative drug.

In Case Study #8, a 32-year-old woman with severe anemia (Hb $4.6\,\text{g/dL}$, reticulocytes $8.3\%$), positive ANA, positive anti-dsDNA, microspherocytes on blood smear, and a positive direct antiglobulin test (DAT) for IgG has what condition?

Warm Autoimmune Hemolytic Anemia (WAIHA) secondary to Systemic Lupus Erythematosus (SLE).