Which forms of thalassemia are compared in the table?

The table compares beta-thalassemia, alpha-thalassemia, delta-beta thalassemia, gamma-delta-beta thalassemia, epsilon-gamma-delta-beta thalassemia, hereditary persistence of fetal hemoglobin, compound thalassemias and rare regulatory forms.

What is the main difference between beta-thalassemia and alpha-thalassemia?

Beta-thalassemia results from defects in the HBB gene and causes reduced beta-chain production, whereas alpha-thalassemia is due to deletions or mutations of the HBA1 and HBA2 genes and reduces alpha-chain production.

What are the most severe forms of thalassemia?

The most severe forms include beta-thalassemia major, which may require regular transfusions, and alpha-thalassemia with loss of all four alpha genes, associated with hydrops fetalis due to Hb Bart’s.

What distinguishes delta-beta thalassemia from beta-thalassemia?

Delta-beta thalassemia affects the delta- and beta-chain cluster. In heterozygotes, HbA is present, HbA2 is low or low-normal and HbF is increased; in biallelic forms, HbA may be markedly reduced or absent.

What is hereditary persistence of fetal hemoglobin?

Hereditary persistence of fetal hemoglobin, or HPFH, is a genetic condition in which HbF production continues beyond the neonatal period, generally without significant anemia or ineffective erythropoiesis.

How is HPFH distinguished from delta-beta thalassemia?

HPFH generally has a normal hematologic profile and does not cause significant anemia. Delta-beta thalassemia instead causes microcytosis and may cause mild anemia, although it is also associated with increased HbF.

Which tests distinguish the different forms of thalassemia?

The main tests are a complete blood count with red blood cell indices, peripheral smear, hemoglobin electrophoresis or HPLC and molecular analysis of globin genes. Rare forms may require extended genetic testing.

What are compound thalassemias?

Compound thalassemias result from coexistence of multiple globin defects, such as alpha and beta variants or the association of thalassemia with structural hemoglobinopathies. The phenotype depends on the interaction among the different abnormalities.

Why is genetic counseling important in thalassemias?

Genetic counseling makes it possible to interpret transmission risk, identify carrier couples and assess the possibility that children may inherit combinations responsible for severe clinical forms.

What is the prognosis of the main forms of thalassemia?

The prognosis is generally excellent in carriers and mild forms, variable in intermediate forms and more challenging in major transfusion-dependent forms. Hydrops fetalis due to Hb Bart’s is the most severe alpha-thalassemia phenotype.

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Comparison table of the main forms of thalassemia

Article written and reviewed by Dr Luca Moretti

This table summarizes the key characteristics of the main forms of thalassemia and related conditions, facilitating comparison of the genetic, clinical and diagnostic differences among these entities. For each form, it reports the genetic cause, essential pathophysiologic mechanisms, typical clinical features, main hematologic and laboratory abnormalities, reference diagnostic tests, prognosis and distinguishing features useful in the differential diagnosis.

Form of Thalassemia	Genetic Defect	Pathogenetic Mechanism	Clinical Features	Laboratory Findings	Prognosis	Distinguishing Features
β Thalassemia (major, intermedia, trait)	Mutations/deletions of the HBB gene (β-globin, chr 11)	Deficient β-chain production and relative excess of α chains; in clinically significant forms, oxidative injury, ineffective erythropoiesis and hemolysis of variable degree	Ranges from severe transfusion-dependent anemia (major) to asymptomatic forms (trait); splenomegaly, bone deformities and iron overload with repeated transfusions	Hypochromic microcytic anemia, variable reticulocytosis, HbA2 often ↑ in trait, HbF ↑ especially in severe forms, circulating erythroblasts mainly in severe forms	Variable: severe and reduced in major forms, excellent in trait	Mediterranean, African and Asian distribution; newborn screening is possible; gene or editing therapies are authorized for selected patients in some jurisdictions
α Thalassemia (silent carrier to major/HbH/Hb Bart’s)	Deletions/mutations of HBA1/HBA2 (α-globin, chr 16)	Alpha-chain deficiency with formation of fetal Hb Bart’s, which has extremely high oxygen affinity, and postnatal HbH, which is unstable and causes oxidative injury and hemolysis	Ranges from asymptomatic silent-carrier status to severe hydrops fetalis due to Hb Bart’s; in hemoglobin H disease, hemolytic anemia and splenomegaly vary in severity and may be severe, particularly in nondeletional forms	Hypochromic microcytic anemia, increased HbH (β tetramers), Hb Bart’s (γ₄) in newborns, and normal or low HbA2	Good in silent-carrier and trait forms; hydrops fetalis due to Hb Bart’s is generally lethal without fetal treatment; prognosis in hemoglobin H disease is generally favorable but varies with genotype and severity	Particularly common in Southeast Asia; HbH disease is often diagnosed in childhood
δβ Thalassemia	Usually deletions of HBD and HBB; rare nondeletional forms with reduced combined expression	Combined δ- and β-chain deficiency, compensation through increased HbF, microcytosis, mild ineffective erythropoiesis	Mild to moderate microcytic anemia, often asymptomatic; phenotype intermediate between trait and HPFH	In heterozygotes: HbA present, HbA2 low or low-normal, and HbF often 5-20%; in biallelic forms HbA is markedly reduced or absent	Generally favorable in heterozygotes; biallelic forms may present as thalassemia intermedia with variable severity	Important in the differential diagnosis with HPFH and beta-thalassemia
γδβ Thalassemia	Deletions of HBG1/HBG2, HBD and HBB, or regulatory abnormalities that functionally inactivate these genes	Silencing of gamma, delta and beta expression from the deleted allele; the normal allele remains functional and the phenotype changes with development	Variable neonatal anemia, sometimes requiring transfusion, with frequent improvement in the first months and possible mild residual microcytosis	Variable profile; after the neonatal period HbA, HbA2 and HbF may all be present and may approach usual values	Very limited data; in the isolated heterozygous forms described, the course is often favorable after the neonatal period but depends on the molecular abnormality	Rare; molecular diagnosis is required, with clinical relevance mainly in the neonatal period and family counseling
εγδβ Thalassemia	Extensive deletions or rearrangements that inactivate the β-globin cluster (HBE1, HBG1/2, HBD, HBB)	Silencing of the beta-globin cluster on the deleted allele, with compensation by the normal allele and possible prenatal or neonatal anemia	Prenatal or neonatal anemia, sometimes severe, with possible hydrops or transfusion requirement; frequent improvement during the first months	Variable fractions: the normal allele produces HbF in the newborn and HbA/HbA2 after switching; the adult profile may be nonspecific	Prognosis depends on perinatal severity and any involvement of contiguous genes	Extremely rare, diagnosed only in specialist centers
Hereditary persistence of fetal hemoglobin (HPFH)	Deletions of the beta-globin cluster or regulatory variants of the HBG1/HBG2 promoters	Inherited persistence of HbF beyond the neonatal period due to altered regulation of the γ→β switch, without anemia or ineffective erythropoiesis in isolated HPFH	Generally asymptomatic when isolated; the phenotype depends on the specific variant and any associated conditions	HbF increased to a degree that varies with the specific variant, with values that may be below 5% in some nondeletional HPFH forms and are often much higher in heterozygous deletional forms; HbA and HbA2 vary by variant, with a generally normal hematologic profile	Generally excellent in isolated HPFH	Must be distinguished from other thalassemias; important in genetic and reproductive counseling
Compound thalassemias and mixed forms	Association of variants in different globin genes or different alleles of the same gene (e.g. α+β, α+δβ, HbS/β-thalassemia, HbE/β-thalassemia)	Sum/interaction of the defects involved, with compensatory or amplified effects on globin-chain balance	Variable anemia and microcytosis; interactions may attenuate or worsen the phenotype	Atypical laboratory pattern, microcytosis, variable HbA2 and HbF, presence of hemoglobin variants	Variable according to the genetic combination and clinical phenotype; reproductive risk depends on parental genotypes	Intermediate phenotypes, complex diagnosis, essential genetic counseling
Regulatory defects and genetic modifiers	Variants in promoters, the LCR and enhancers, or trans-acting modifiers (BCL11A, KLF1, HBS1L-MYB)	Heterogeneous effect on globin expression; some defects cause a specific thalassemia, while others modify HbF or the phenotype of an associated disease	Variable, often asymptomatic; atypical hemoglobin fractions or modification of the severity of an associated hemoglobin disorder may occur	HbA2, HbF and red-cell indices vary according to the specific variant	Dependent on the specific variant and associated disease	Molecular testing must be selected according to the phenotype; genetic counseling depends on locus, phase and inheritance

For γδβ and εγδβ forms and for some compound combinations, data derive from family series or rare reports: the profiles shown are indicative and must not be interpreted as invariant phenotypes.

Understanding the differences among the main forms of thalassemia and related conditions is essential for correct diagnosis, clinical management, genetic counseling and prevention of severe forms. For detailed information, see the individual dedicated pages.

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