Screening for the β-Thalassemia Trait in Iron-Deficient Children: Are HbA2 Measurements Reliable?
—According to this study, more than half of children with the beta (β)-thalassemia trait and iron deficiency had hemoglobin A2 (HbA2) levels in the diagnostic range, supporting the potential of HbA2 as a screening tool for this inherited blood disorder.
The inherited blood disorder beta (β)-thalassemia, which can cause severe anemia, is treated with iron-chelation therapy and transfusions. As the gene mutations that trigger β-thalassemia are recessive, screening for potential carriers can inform family planning for individuals with a family history of the disorder. Worldwide estimates for carriers of these mutations range from 1% to 5%, with cases of β-thalassemia becoming more common in North America and Europe as a result of migration.1,2
Heterozygosity (called β-thalassemia trait, β-thalassemia carrier, or β-thalassemia minor) is typically screened by measuring hemoglobin A2 (HbA2) levels rather than by DNA sequencing.2 Variants in the β-globin gene that cause the β-thalassemia trait result in reduced HbA, which is comprised of a tetramer of 2 α- and 2 β-globin chains by 6 months of age. Consequently, the HbA2 form, comprised of two α- and two δ-globin chains, is elevated relative to the HbA form in individuals with the β-thalassemia trait.2
While most people with the β-thalassemia trait have elevated HbA2, false negatives can occur when HbA2 does not reach the diagnostic threshold. A recent study found that approximately one-third of individuals with borderline HbA2 levels have mutations in β-globin or other genes.3
HbA2 levels and iron deficiency
Another concern is the effect of iron deficiency on HbA2 levels, say the researchers behind a new study published in the American Journal of Clinical Pathology.4 While older studies have sometimes shown that iron deficiency reduces HbA2, some more-recent studies have found that HbA2 is usually elevated in adults, even in the setting of iron deficiency. Therefore, the authors argue, HbA2 testing is still an appropriate screening tool.
In light of these reports in adults, the pair of investigators who designed the new study asked whether HbA2 levels were affected by iron deficiency in children. “Could a pediatrician fail to diagnose an iron-deficient β-thalassemia carrier?” they wrote.4
To answer this question and others, the investigators assessed HbA2 levels in 213 children (107 girls and 106 boys) in Turkey with the β-thalassemia trait with or without iron deficiency.4
Study design and data
Data from the study were obtained from the medical records of patients followed up at the Pediatric Hematology Department of Gazi University Medical Faculty, in Ankara, Turkey.4 Participants had received a diagnosis of β-thalassemia trait from tests performed between January 2015 and December 2019. The children were a median age of 6 years (range 1 to 17 years).
The diagnostic criteria for participants with normal iron status of ≥15 ng/mL and transferrin saturation of ≥15% were microcytosis with a mean corpuscular volume of <80 fL, hypochromia with a mean corpuscular hemoglobin of <25 pg per cell, and erythrocytosis with a red blood cell count of >5 x 1012/L. HbA2 was measured by high-performance liquid chromatography, with HbA2 ≥3.5% considered to be consistent with the β-thalassemia trait. Those with an HbA2 percentage <3.5% received a diagnosis of β-thalassemia trait if a parent had similar red blood cell indices.
For the analysis in the new study, iron deficiency was defined as ferritin <15 ng/mL. Comparisons between participants with and without iron deficiency were also carried out using transferrin saturation <15% as an alternative definition of iron deficiency.
What the researchers discovered
Among the 213 children with the β-thalassemia trait, 51 (23.9%) had iron deficiency based on ferritin concentration <15 ng/mL. The median (standard deviation [SD]) iron concentration of children with iron deficiency was 53.0 (SD 28.7) μg/dL, compared with 74.1 (29.8) μg/dL for those with iron sufficiency. When iron deficiency was defined as transferrin saturation <15%, iron deficiency was diagnosed in 55 (25.8%) children.
HbA2 levels were lower in participants with iron deficiency but usually in the diagnostic range for the β-thalassemia trait. HbA2 levels were ≥3.5% in 28 of 51 (54.9%) participants with iron deficiency. The median HbA2 level was 4.1% (SD 1.5%) in those with iron deficiency and 4.9% (SD 1.4%) in those without iron deficiency (P < .007).
Thirty of the 55 (54.5%) children with low transferrin saturation had HbA2 levels of at least 3.5%. The median HbA2 level in iron-deficient participants defined by transferrin saturation was the same as when iron deficiency was defined by ferritin concentration (4.1% [SD 1.5%]).
A useful marker for now, but more research is needed
The investigators also evaluated the iron status of children with HbA2 below the diagnostic threshold and found that many were not iron-deficient. Of the 213 children in the study, 67 (31.5%) had HbA2 levels below the 3.5% threshold for diagnosis of the β-thalassemia trait. Out of these 67 patients, 43 (64.2%) were iron-sufficient, with a median HbA2 of 2.7% (SD 0.3%).
Noting that HbA2 was elevated in more than half of children with iron deficiency in their study, the researchers wrote, “This finding underscores the reliability of HbA2 as a diagnostic tool for ß-thalassemia trait screening, even in the presence of iron deficiency.”4
The investigators pointed out that HbA2 levels below the diagnostic threshold in individuals with the β-thalassemia trait may have causes other than iron deficiency. For example, variants in δ-globin genes may also be associated with reduced HbA2.5 “Therefore, it would not be correct to attribute low HbA2 levels to iron deficiency without studying variation analysis,” they explained.4
However, because genetic testing is not universally available, the investigators recommend taking additional steps when HbA2 levels are borderline. “If iron deficiency is present and HbA2 levels are found to be below 3.5%, continue the evaluation for a family study without distraction,” they wrote. “Low HbA2 levels in these cases may be related to the underlying thalassemia variation rather than iron deficiency because iron deficiency does not significantly reduce HbA2 levels in [the] β-thalassemia trait.”4
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