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Myelodysplastic Syndromes (MDS)

Definition

1. Cytopenias from ineffective hematopoiesis (the marrow is typically hypercellular yet peripheral blood counts are low - a hallmark paradox)
2. High risk of transformation to acute myeloid leukemia (AML)

• Harrison's Principles of Internal Medicine 22E (2025), Chapter 107
• Robbins & Kumar Basic Pathology, p. 405

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Epidemiology

• Incidence: ~15,000 new cases/year in the United States - roughly as common as AML
• Median age at diagnosis: seventh to eighth decade (most patients are 50-80 years old)
• MDS in childhood is rare and implies an underlying genetic disease (e.g., Down syndrome, Fanconi anemia, GATA2 mutations)
• More common in males
• Incidence has been underestimated due to varying diagnostic criteria

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Etiology and Risk Factors

• Prior chemotherapy (especially alkylating agents, topoisomerase II inhibitors)
• Prior ionizing radiation therapy
• These therapy-related cases share similar recurrent chromosomal abnormalities with primary MDS

• Germline GATA2 mutations (MonoMAC syndrome - susceptibility to viral, mycobacterial, and fungal infections, deficient monocytes, NK cells, B cells)
• Germline RUNX1 mutations (high risk of MDS/leukemia, often preceded by years of thrombocytopenia)
• Fanconi anemia, telomeropathies (TERC, TERT mutations)
• Constitutional SAMD9/SAMD9L mutations

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Pathogenesis

Molecular Mutations (three major categories)

Chromosomal Abnormalities

• Monosomies: -5, -7
• Deletions: del(5q), del(7q), del(20q)
• Trisomy: +8
• The 5q- deletion causes heterozygous loss of a ribosomal protein gene (RPS14), which mimics constitutional red cell aplasia - this is the mechanistic basis for lenalidomide sensitivity

Immune Dysregulation

• In lower-risk MDS, an immune pathophysiology may be important - cytopenias can respond to immunosuppressive therapy (as in aplastic anemia)
• The role of the hematopoietic stem cell niche and microenvironment remains incompletely understood

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Classification (WHO 5th Edition / ICC 2022)

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Clinical Features

Symptoms

• Anemia dominates the early course: fatigue, weakness, dyspnea, pallor
• At least half of patients are asymptomatic at diagnosis - discovered incidentally on CBC
• Bleeding symptoms (due to platelet dysfunction despite adequate counts)
• Increased susceptibility to infection (neutropenia + neutrophil dysfunction)
• Fever and weight loss are more characteristic of myeloproliferative rather than myelodysplastic disease

Physical Examination

• Signs of anemia (pallor, tachycardia)
• ~20% have splenomegaly
• Accompanying autoimmune syndromes occur (Sweet's syndrome, vasculitis)
• VEXAS syndrome should be considered in MDS with coexisting inflammatory disease (caused by somatic UBA1 mutations)

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Laboratory Findings

Peripheral Blood

• Anemia (usually macrocytic) - present in most cases
• Bicytopenia or pancytopenia in advanced disease
• Large platelets lacking granules, functionally abnormal
• Pseudo-Pelger-Huet neutrophils (hyposegmented nuclei), hypogranulated neutrophils, Döhle bodies
• Circulating myeloblasts (correlates with marrow blast %)
• WBC usually normal or low (except CMML where monocytosis occurs)
• Associated PNH clone may be present

Bone Marrow

• Usually normocellular or hypercellular (paradoxically, despite peripheral cytopenias)
• ~20% are hypocellular - can be confused with aplastic anemia
• Dysplastic changes in all three lineages (see morphology below)
• Myeloblasts <20% by definition (≥20% = AML)

Bone Marrow Morphology

Cytogenetics & Molecular Testing

• Conventional karyotype: abnormal in ~50% of primary MDS, ~80% of therapy-related MDS
• Next-generation sequencing is now routinely performed - detects mutations in >90% of MDS cases
• Testing for SF3B1, SRSF2, TET2, ASXL1, DNMT3A, TP53, RUNX1, etc.

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Prognosis: IPSS-R Scoring

• Median overall survival: 9-29 months (varies by risk group)
• AML transformation: 10-40% of patients
• Molecular IPSS (IPSS-M) incorporating mutation data now provides better prognostic discrimination

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Treatment

Supportive Care (all risk groups)

• Red cell transfusions for symptomatic anemia
• Platelet transfusions for significant bleeding
• Iron chelation (deferoxamine, deferasirox) for transfusion-dependent patients with iron overload
• G-CSF for severe neutropenia with infections (typically not routinely given)
• Growth factors: Erythropoiesis-stimulating agents (ESAs - epoetin alfa, darbepoetin) for patients with low endogenous EPO levels; response rate ~25-40%

Lower-Risk MDS (IPSS-R Very Low/Low/Intermediate)

• Highly effective in MDS with 5q- syndrome
• A high proportion become transfusion-independent with near-normal hemoglobin
• Cytogenetics normalize in many patients
• Administered orally; toxicities include myelosuppression and DVT/PE risk

• Antithymocyte globulin (ATG) + cyclosporine
• Anti-CD52 monoclonal antibody alemtuzumab
• Especially effective in patients <60 years with HLA-DR15 positivity and a PNH clone

• FDA-approved for ESA-relapsed/refractory lower-risk MDS
• Phase 3 IMerge trial (Lancet, 2024) demonstrated transfusion independence vs placebo [PMID 38048786]

Higher-Risk MDS (IPSS-R High/Very High)

• Azacitidine (5-azacytidine): subcutaneous, daily ×7 days every 28 days; improves blood counts and survival vs best supportive care; response in ~50% of patients; FDA-approved for all MDS subtypes; responses are dependent on continued administration
• Decitabine: IV, various schedules (3-10 days); response in 30-50%, duration ~1 year
• Oral decitabine-cedazuridine (Inqovi): FDA-approved oral formulation; Phase 3 ASCERTAIN trial (Lancet Haematol, 2024) showed non-inferiority to IV decitabine [PMID 38135371]

• Phase 1/2 data from Garcia et al. (Blood, 2025) show promising activity for treatment-naive high-risk MDS [PMID 39652823]
• Not yet standard of care but incorporated into clinical trials

Curative Intent: Allogeneic Hematopoietic Stem Cell Transplantation (allo-HSCT)

• The only known curative treatment for MDS
• Survival ~50% at 3 years in selected cohorts (improving with better supportive care and donor matching)
• Matched unrelated donor results are similar to sibling donor results
• Barriers: Advanced age of most patients, comorbidities, donor availability, patient preferences
• Only a small proportion of MDS patients ultimately undergo transplantation
• Reduced-intensity conditioning (RIC) now extends eligibility to older patients (though higher relapse risk)
• Transplant timing dilemma: high-risk patients most need it but tolerate it least; low-risk patients tolerate it best but may do well for years with less aggressive therapy
• Pre-transplant HMA therapy (bridging) is common; prior therapy does not appear to significantly harm outcomes

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MDS vs. AML Distinction

• AML: ≥20% myeloblasts in BM or peripheral blood
• MDS with excess blasts: 5-19% BM blasts (high leukemic transformation risk)
• The ICC (2022) introduced MDS/AML (≥10% blasts) to recognize the biological continuum - these cases often have similar outcomes regardless of which side of the 20% threshold they fall on

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Key Associations to Remember

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Recent Developments (2024-2026)

• Oral decitabine-cedazuridine (Inqovi) confirmed non-inferior to IV decitabine in Phase 3 ASCERTAIN trial - improves convenience significantly [PMID 38135371]
• Imetelstat (telomerase inhibitor) Phase 3 trial confirmed efficacy in lower-risk, ESA-refractory MDS with transfusion-independence benefit [PMID 38048786]
• Venetoclax + azacitidine showing promising Phase 1/2 data for high-risk MDS (Blood, 2025) [PMID 39652823]
• Both WHO 5th Edition (2022) and ICC (2022) now use molecular markers to define subtypes - the era of purely morphologic classification is ending
• IPSS-M (molecular IPSS) provides refined prognostication incorporating gene mutation data

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Thalassemia

Definition

• The name comes from the Greek thalassa ("sea"), reflecting the original description in Mediterranean populations.
• ~500 unique thalassemia-causing mutations are known; clinically, β-thalassemia and α-thalassemia are the most important.

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Genetics and Normal Hemoglobin Structure

• HbF (fetal) = α₂γ₂ - elevated in thalassemia, partially compensatory
• HbA₂ = α₂δ₂ - elevated in β-thalassemia trait (diagnostic marker)
• HbH = β₄ tetramers - form in α-thalassemia when α chains are severely reduced
• Hb Bart's = γ₄ tetramers - form in severe α-thalassemia in fetal life

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Epidemiology

• Estimates: 1-5% of the world's population carries a thalassemia mutation; in some locales, most people are carriers
• Common in regions where Plasmodium falciparum malaria was (is) endemic: the Mediterranean basin, Middle East, tropical Africa, Indian subcontinent, Southeast Asia
• Evolutionary explanation: Heterozygous carriers are protected from severe P. falciparum malaria - this drove the mutation to polymorphic frequencies
• ~40,000 β-thalassemia patients are born yearly worldwide
• About 30% of African Americans carry the common -α³·⁷ chromosome (single-gene α-thalassemia)
• HbH disease and Hb Bart's hydrops fetalis are most common in southern China and Southeast Asia

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Part 1: β-Thalassemia

Molecular Pathogenesis

• β⁰ mutations: Completely prevent β-globin accumulation
• β⁺ mutations: Cause reduced (but detectable) β-globin synthesis

1. Excess α-chains accumulate and precipitate within erythroblasts → membrane lipid oxidation and damage
2. Ineffective erythropoiesis (intramedullary destruction of erythroid precursors) - the predominant cause of anemia
3. Reduced deformability and phosphatidylserine exposure → extravascular and intravascular hemolysis of circulating cells
4. Severe anemia → compensatory erythroid marrow expansion → skeletal deformities
5. Increased intestinal iron absorption + transfusional iron → iron overload in liver, heart, and endocrine organs

Classification of β-Thalassemia

Clinical Features of β-Thalassemia Major (Cooley's Anemia)

• Severe hemolytic anemia - jaundice, pallor, fatigue from early childhood
• Massive hepatosplenomegaly - extramedullary hematopoiesis in liver, spleen
• Skeletal deformities from marrow expansion:
  • "Chipmunk" facies (maxillary hypertrophy)
  • Frontal bossing
  • "Hair-on-end" appearance on skull X-ray (classic radiographic finding)
  • Pathological fractures
• Growth retardation, delayed puberty
• Iron overload complications (even before transfusions, from increased gut absorption):
  • Cardiomyopathy (leading cause of death)
  • Cirrhosis, hepatic failure
  • Endocrinopathies: diabetes, hypogonadism, hypothyroidism, hypoparathyroidism, adrenal insufficiency
  • Osteoporosis

Blood Film in Thalassemia Intermedia

Diagnosis of β-Thalassemia

• CBC: Microcytic (low MCV), hypochromic anemia; normal/low reticulocytes (ineffective erythropoiesis); anisocytosis, poikilocytosis, target cells, basophilic stippling
• Hemoglobin HPLC/electrophoresis:
  • Trait: HbA₂ elevated (4-6%) - the diagnostic hallmark
  • Major: predominantly HbF (>60%), markedly reduced or absent HbA, elevated HbA₂
• Serum ferritin and iron studies: Iron overload (NOT deficiency - do not give iron!)
• Bone marrow: Erythroid hyperplasia, ineffective erythropoiesis
• DNA analysis: Required before genetic counseling and antenatal diagnosis to identify the specific mutation
• Must exclude iron deficiency (which also causes microcytosis but with low ferritin, low HbA₂)

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Part 2: α-Thalassemia

Molecular Pathogenesis

• α-thalassemia is most often caused by deletions of α-globin genes (rather than point mutations)
• Each chromosome 16 has two α-globin genes → diploid individuals have 4 α-globin genes total
• Severity is proportional to the number of deleted/non-functional α-globin genes

Classification of α-Thalassemia

• Asians: commonly carry the --/αα haplotype (both deletions on one chromosome) → at risk for HbH disease and hydrops in offspring
• Africans: commonly carry -α/-α (one deletion per chromosome) → very rarely produce severely affected offspring even when both parents are carriers

Clinical Features of HbH Disease

• Moderately severe hemolytic anemia (resembles β-thalassemia intermedia)
• HbH (β₄ tetramers) has extremely high O₂ affinity → delivers little O₂ to tissues → disproportionate tissue hypoxia
• HbH oxidizes and precipitates → intracellular inclusions → spleen sequestration
• Hepatosplenomegaly, jaundice, thalassemic facies, and growth impairment in 20-50% of cases
• Iron loading occurs but is less severe than in β-thalassemia
• Diagnosis: HbH inclusions on brilliant cresyl blue stain; HPLC shows traces to 40% HbH in adults

Clinical Features of Hb Bart's Hydrops Fetalis

• Deletion of all 4 α-globin genes → no functional hemoglobin can form
• Hb Bart's (γ₄) has extremely high O₂ affinity - unable to deliver O₂ to fetal tissues
• Third trimester fetal distress → severe pallor, generalized edema (hydrops), massive hepatosplenomegaly, ascites
• Without intrauterine transfusion: fatal in utero or shortly after birth
• With intrauterine transfusion + perinatal intensive care: survival possible, but ~40% have growth retardation, ~20% neurodevelopmental delay
• Maternal complications: preeclampsia, polyhydramnios, antepartum hemorrhage, difficult delivery
• Prevention (antenatal screening + diagnosis) is the best approach

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Part 3: Treatment

Supportive Care

• Every 2-4 weeks with goal pretransfusion Hb of 9-10.5 g/dL
• Suppresses ineffective erythropoiesis, prevents skeletal deformities
• Must be started early and continued lifelong without interruption
• Use leukodepleted, antigen-matched (to minimize alloimmunization) packed red cells

• Prevents accumulation of toxic iron in heart, liver, and endocrine organs
• Indicated when ferritin is consistently elevated (generally >1,000-2,500 mcg/L)
• Three chelating agents available:

• A Cochrane systematic review (2023) examined calcium channel blockers for preventing thalassemia cardiomyopathy alongside iron chelation [PMID 37975597]

Improving Ineffective Erythropoiesis

• Fusion protein (activin type IIB receptor extracellular domain + IgG Fc)
• Binds TGF-β superfamily ligands → reduces Smad2/3 signaling → enhances late-stage erythropoiesis
• Given subcutaneously 1 mg/kg every 3 weeks
• Phase 3 BELIEVE trial: 33% reduction in transfusion requirements in transfusion-dependent β-thalassemia
• FDA-approved for adults with transfusion-dependent β-thalassemia

• Increases HbF synthesis; used in some non-transfusion-dependent cases
• More effective in sickle cell disease but used as adjunctive therapy in some thalassemia patients

• Reduces hypersplenism and transfusion requirements in selected patients
• Increased risk of overwhelming post-splenectomy infection and thrombosis - performed less frequently now

Curative Treatments

• The only established curative treatment
• Best results in young children (<7 years), without organ damage from iron overload
• Class I patients (no hepatomegaly, no fibrosis, adequate chelation): >90% event-free survival
• Matched unrelated donor transplants now yield outcomes similar to sibling donors
• HbH disease: transfusions not usually needed; allo-HSCT rarely indicated

• CRISPR/Cas9 - Casgevy (exagamglogene autotemcel): Edits the BCL11A gene (erythroid enhancer) to reactivate γ-globin production → induces HbF

  • 90% of 52 patients aged 12-35 with transfusion-dependent β-thalassemia achieved transfusion independence with total Hb ≥12 g/dL and HbF ≥10 g/dL

• Lentiviral gene therapy - Zynteglo (betibeglogene autotemcel): Adds a functional HbA^T87Q^ β-globin gene; best results in non-β⁰/β⁰ genotypes
• A 2026 systematic review of CRISPR trials confirmed sustained responses in thalassemia and sickle cell disease [PMID 39794549]

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Screening, Counseling, and Antenatal Diagnosis

• Carrier screening: CBC with red cell indices (microcytosis + normal iron → suspect thalassemia trait) → HbA₂ quantification → DNA mutation analysis
• Genetic counseling: Essential before conception or early in pregnancy when both partners are carriers
• Antenatal diagnosis: Chorionic villus sampling (10-12 weeks) or amniocentesis for DNA analysis; can identify β⁰/β⁰ or --/-- fetuses
• Effective screening programs have substantially reduced new case births in Italy, Cyprus, and other high-prevalence regions

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Key Distinguishing Features: β- vs. α-Thalassemia Trait

Important: Never treat microcytic anemia due to thalassemia trait with iron supplements - iron stores are normal or elevated. Always exclude iron deficiency first with ferritin/serum iron.

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Summary: Clinical Spectrum at a Glance

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Hemoglobin Electrophoresis

Purpose

• Diagnose hemoglobinopathies (sickle cell disease, HbC, HbE, HbD, etc.)
• Diagnose thalassemias (especially β-thalassemia trait via elevated HbA₂)
• Newborn screening for clinically significant Hb disorders
• Confirm sickle cell preparations (all positive sickle tests must be confirmed by electrophoresis or isoelectric focusing)

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Normal Hemoglobin Components

Normal adult pattern: >97% HbA, <3% HbA₂, <1% HbF, no other bands.

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Methods

1. Alkaline Electrophoresis (pH 8.4-8.6) - Cellulose Acetate

← Cathode (-) ←————————————————————————————→ Anode (+)

  C   A₂  E   O          S   D   G        F   A   Bart's   H
  |___|___|___|           |___|___|        |___|   |_________|
  (C position)           (S position)     (F pos.) (fast Hbs)

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2. Acid Electrophoresis (pH 6.0-6.2) - Citrate Agar

Mnemonic for acid gel: "AFSC" - HbA, HbF, HbS, HbC are the four major bands visible; variants that co-migrated with S at alkaline pH now move toward A.

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3. High Performance Liquid Chromatography (HPLC) - Current Gold Standard

• Accurately quantifies HbA₂ and HbF (electrophoresis cannot do this reliably)
• Higher throughput and automation
• Also quantifies HbA1c (glycated hemoglobin for diabetes monitoring)
• Better sensitivity for small peaks

• HbE and HbA₂ co-elute (similar retention times) - cannot easily separate
• HbC and HbO-Arab co-elute - cannot easily separate
• Capillary zone electrophoresis resolves HbE from HbA₂ (important for accurate HbA₂ quantification in populations where HbE is common)
• Does NOT help separate HbD from HbG and Hb-Lepore

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4. Isoelectric Focusing (IEF)

• Separates Hb variants by their isoelectric point (pI)
• Widely used in newborn screening programs (more sensitive than electrophoresis for detecting HbF fractions)
• Part of mandatory newborn screening in many U.S. states

5. Capillary Zone Electrophoresis (CZE)

• Separates Hb variants in a capillary tube at alkaline pH
• Can accurately quantify HbA₂ even in the presence of HbE (unlike HPLC)
• Increasingly used as a standalone confirmatory method

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Interpreting Hb Electrophoresis Results

Normal Adult Pattern

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β-Thalassemia Trait (Minor)

• HbA₂ ≥4% is the diagnostic hallmark of β-thalassemia trait
• Borderline 3.1-3.9%: consider heterozygous KLF1 mutations or concomitant δ-globin mutation
• Must be quantified by HPLC (densitometric scanning of gel is inaccurate)
• False causes of elevated HbA₂: megaloblastic anemia, hyperthyroidism, some unstable Hbs, HIV in women

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β-Thalassemia Major

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α-Thalassemia Trait

• Electrophoresis is typically normal in α-thalassemia trait
• Diagnosis is confirmed by DNA analysis (gene deletion testing)

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Sickle Cell Trait (HbAS)

• Band in S position on alkaline electrophoresis → confirm with acid gel (stays at S)
• Sickle screen (sodium metabisulfite / Sickledex) must be confirmed by electrophoresis
• More HbA than HbS (unlike sickle cell disease where no HbA is present)

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Sickle Cell Disease (HbSS)

• Single S band at alkaline pH; confirmed at S position on acid gel
• Sickle screen positive

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HbSC Disease

• Two bands: one at S position, one at C position (alkaline gel)
• Acid gel: S stays at S; C stays at C - confirms both
• Clinical severity intermediate between sickle trait and HbSS

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HbS/β⁰-Thalassemia

• Clinically indistinguishable from HbSS
• Key distinguishing feature: elevated HbA₂ (not seen in HbSS)
• Newborn screen: FS pattern

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HbS/β⁺-Thalassemia

• HbA IS present (unlike HbSS and HbS/β⁰), but HbS > HbA
• Milder clinical course than HbSS
• Newborn screen: FSA pattern

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HbC Disease (HbCC)

• HbC = β⁶Glu→Lys (vs HbS = β⁶Glu→Val, same position, different amino acid)
• Alkaline gel: band at C position; acid gel: stays at C position
• Mild hemolytic anemia; numerous target cells on blood smear
• HbC trait (AC): ~60% HbA + ~40% HbC; usually asymptomatic

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HbE Trait (AE)

• Alkaline gel: band at C position (same as HbC)
• Acid gel: band at A position (migrates away from HbC - distinguishes E from C)
• HbE/β-thalassemia: one of the most common severe hemoglobin disorders worldwide (Southeast Asia)
• Capillary electrophoresis needed to separate HbE from HbA₂ for accurate quantification

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Neonatal Hemoglobin Electrophoresis Patterns

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Summary: What Each Method Can and Cannot Do

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Clinical Approach to Microcytic Anemia with Hb Electrophoresis

CBC shows microcytic hypochromic anemia
                    ↓
    Check serum ferritin / iron studies
                    ↓
   ┌─── Low ferritin ──→ Iron deficiency anemia (treat iron first)
   │
   └─── Normal/high ferritin ──→ Hb electrophoresis + HPLC
                    ↓
    ┌── Elevated HbA₂ (≥4%) ──→ β-Thalassemia trait
    │
    ├── Normal HbA₂, normal pattern ──→ α-Thalassemia trait (confirm by DNA)
    │
    ├── HbS present ──→ Sickle syndromes (classify by pattern above)
    │
    ├── HbC or HbE present ──→ See specific diagnosis above
    │
    └── Markedly elevated HbF, no/little HbA ──→ β-Thalassemia major

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