ABSTRACT
Haemoglobinopathy testing is performed for carrier screening and evaluation of microcytic anaemia. We evaluated the effectiveness of thalassaemia screening tests at our institution and suggest ways of improving the testing algorithm. Materials and Methods: A total of 10,084 non-antenatal and 11,364 antenatal samples with alkaline gel electrophoresis (AGE), capillary electrophoresis (CE), haemoglobin H (HbH) inclusion test, mean corpuscular haemoglobin (MCH) and mean corpuscular volume (MCV) were retrospectively reviewed. A subgroup of 187 samples with genetic testing was correlated with HbH inclusions and MCH/MCV. The effect of iron deficiency on percentage haemoglobin A2 (HbA2) was studied. Results: HbH inclusion test showed low sensitivity of 21.43% for α-thalassaemia mutations but higher sensitivity of 78.95% for --SEA deletion. Byreceiver operating characteristic (ROC) analysis, MCH ≤28 pg or MCV ≤80 fl for non-antenatal samples and MCH ≤27 pg or MCV ≤81 fl for antenatal samples had >98% sensitivity for HbH inclusions. Above these thresholds, the probability that HbH inclusions would be absent was >99% (negative predictive value [NPV] >99%). MCH ≤28 pg had 100% sensitivity (95% CI 95.63%-100%) for α-thalassaemia mutations and 97.68% calculated NPV in the antenatal population. Detection of haemoglobin variants by CE correlated highly with AGE (99.89% sensitivity, 100% specificity). Severe iron deficiency reduced HbA2 in haemoglobin E (P <0.001) and α-thalassaemia (P = 0.0035), but not in β-thalassaemia. Conclusion: MCH/MCV thresholds have adequate sensitivity for α-thalassaemia in the antenatal population, and genotyping plays an important role as HbH inclusion test shows low sensitivity. CE without AGE, may be used as initial screening for haemoglobin variants. Our study provides contemporary data to guide thalassaemia screening algorithms in Singapore.
Haemoglobinopathies are inherited disorders of haemoglobin (Hb) in which the genetic abnormality leads to reduced synthesis of normal globin chains (α- and β-thalassaemia) or functional changes in haemoglobin (haemoglobin structural variants).1 The haematological parameters of patients with thalassaemia differ widely, ranging from asymptomatic carriers to severe anaemia requiring regular blood transfusion. Globin chain imbalance results in ineffective erythropoiesis, anaemia and microcytosis, the degree of imbalance translating into clinical severity. Deletion of 1 or 2 of 4 alpha-globin genes (–α/αα, – –/αα, –α/–α) causes α-thalassaemia trait; deletion of 3 alpha-globin genes –α/– – or deletions in combination with non-deletional mutation (e.g. ααCS/– –) leads to haemoglobin H (HbH) disease; while deletion of all 4 alpha-globin genes (– –/– –) causes Barts hydrops fetalis, a fatal condition in-utero.2 Alpha0 denotes 2 gene deletions in cis (– –) while alpha+ denotes 1 gene deletion (–α). β-thalassaemia trait arises from inheritance of 1 β-thalassaemia allele, whereas homozygous inheritance of 2 β-thalassaemia alleles leads to thalassaemia major or a moderate form termed ‘thalassaemia intermedia’. In Southeast Asia and Singapore, compound heterozygosity for haemoglobin E (HbE) (HBB:c.79G>A, β26(B8)Glu>Lys) and β-thalassaemia is a common cause of thalassaemia intermedia.
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