Use of HbA₂ As a Discriminator for S/Beta Thalassaemia in a Nigerian Setting,

Abstract 4206

An integral part of an EU-UNDP funded pilot sickle cell screening project, was the installation, in Abuja (Federal Capital Territory) Nigeria, June 2010, of a High Performance Liquid Chromatography (HPLC) instrument. Prior to the installation, haemoglobinopathy screening was carried out using only unstained paper electrophoresis. Minor bands were difficult to visualise and the proportions of haemoglobins were not measured. Full blood count (FBC) data was also not routinely available. Therefore awareness of both alpha and beta thalassaemia was low as was the implications of coinheritance of beta thalassaemia with haemoglobin (Hb) S.

Stratified community surveys were carried out with samples collected as Guthrie card blood spots; children aged less than 6 months, and whole blood; children aged between 6 months and 5 years. Blood spot samples were analysed using newborn sickle cell screening reagents and whole blood samples using beta thalassaemia short reagents, the later allowing accurate Hb A₂ and Hb F measurement. Samples were processed on a Biorad^™ Classic (instrument and reagents, Biorad, Hercules, CA.). Reporting algorithms were defined for both reagent sets. Using the beta thalassaemia reagents, Hb S/beta thalassaemia was considered when Hb S was predominant, Hb A absent or significantly reduced and the Hb A₂>5.

Over 10,000 samples were analysed, 410 had sickle cell disease, of which 370 were transported to London for further analysis, 70 of these had an Hb A₂>5%, mean and range 5.9(5.1 – 9.1)%. To validate the algorithm 31 samples (Hb A₂ 3.5 – 7.4%), were selected for beta gene sequencing (Mai et al, 2004) and/or PCR analysis for the 7 common alpha gene deletions (3.7kb, 4.2kb, SEA, MED, THAI, FILL, 20.5kb and triplicated alpha gene locus, anti 3.7kb), (Chong et al, 2000, Wang et al, 2003). Five samples had an Hb A₂of 3.5 – 4.9%, 2 with Hb A present (23 and 25%), all were Hb SS, 4 negative for alpha thalassaemia deletions and 1 heterozygous for the alpha 3.7kb deletion. Sixteen samples had an Hb A₂ of 5.0 – 5.9%, all were Hb SS, 5 negative for alpha thalassaemia deletions and 11 heterozygous for the alpha 3.7kb deletion. Seven samples had an Hb A₂ of 6.0 – 6.9%, 6 were Hb SS, 1 negative for alpha thalassaemia deletions, 1 heterozygous and 4 homozygous for the alpha 3.7kb deletion. One, Hb A₂ 6.9%, was a compound heterozygote for Hb S/ beta zero codon 106/107(+G) mutation. Three samples had an Hb A₂ of 7.0 – 7.4%, review of chromatograms indicated that all showed poor chromatography due to lack of separation between Hb A₂ and Hb S or shoulders to the left of the Hb A₂ peak. All were Hb SS and heterozygous for the alpha 3.7kb deletion, with 1 also positive for the triplicated alpha globin gene, anti 3.7kb. Four other samples with Hb A₂ >7% also showed poor chromatography, but were insufficient for molecular testing.

The results indicate that Hb A₂values >5% were mainly due to co-existing alpha thalassaemia with 76% of those tested, positive for the 3.7kb deletion. Poor chromatography was also a contributor particularly at the higher Hb A₂ levels. During the initial stages of the project air conditioning failure caused major temperature fluctuations on overnight runs, this problem was resolved, improving chromatography. One case of S/beta zero thalassaemia was detected confirming the presence in this population. The results suggest that the Hb A₂ may be considered as a discriminator for S/beta thalassaemia screening in this setting, increasing the algorithm Hb A₂ level to >6.0% may improve specificity and reduce the number of false positives, this warrants further investigation. Future aims of the project for children over 6 months of age, include initiating routine FBC analysis and testing parental samples as a cost effective means of confirming suspected cases.