A Multicentre Trial of the Effectiveness of ζ Globin Enzyme Linked Immunosorbent Assay (ELISA) and Hb H Inclusion Body Screening for the Detection of α Thalassemia Trait.
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AbstractThe most serious form of α thalassemia, Hb Barts hydrops fetalis, results in fetal death and dangerous obstetric complications. Syndromes containing two α gene deletions on the same chromosome (in cis), e.g. α thalassemia trait and Hb H disease, are carriers of this condition. The most common cis deletion is the Southeast Asian (--SEA/) deletion. The gold standard diagnosis for carriers is DNA investigation, a technique only available in specialized laboratories. The current routine screening test, Hb H screen, is considered insensitive, observer dependant and time consuming. We present results of a prospective Ontario multicentre study investigating the effectiveness of a new ζ globin ELISA and a standardized Hb H screen compared to DNA testing for the detection of Hb Barts hydrops fetalis carriers. Study testing included CBC, Hb variant detection, Hb A2 and Hb F quantitation, a standardized Hb H inclusion body screen, ferritin level, free erythrocyte protoporphyrin, gap PCR and ζ globin ELISA. The standardized Hb H screen utilized commercially prepared brilliant cresyl blue incubated for one hour at 37°C followed by microscopic examination for Hb H, two minutes each of two blood films. The calculated sensitivities and specificities for the detection of various genotypes of α thalassemia are noted in Table 1. Of 102 carriers 27 (26.5%) had a concomitant cause of microcytosis. Results show ζ globin ELISA is highly sensitive and specific for SEA α thalassemia trait but is less effective than Hb H screening in detecting SEA Hb H disease, and is of no value in the detection of non-SEA α thalassemia trait and non-SEA Hb H disease. Neither test is of value in detecting single α globin gene deletions. The standardized Hb H screen showed better than expected performance. With all carrier genotypes considered Hb H screen and ζ globin ELISA displayed equal sensitivity and specificity. We conclude that a positive ζ globin ELISA is diagnostic of the SEA deletion, that combined use of both a standardized Hb H screen and ζ globin ELISA improves the sensitivity of Hb Barts hydrops fetalis carrier detection from 0.71 to 0.91 with no loss of specificity, combined testing should be standard practice for α thalassemia testing in routine laboratories and that detection of an alternate cause of microcytosis does not exclude Hb Barts hydrops fetalis carriers and should not be used for this purpose.Table 1 Sensitivities and specificities. Genotypes Hb H Inclusion Bodies ζ globin ELISA ζ globin ELISA and Hb H Inclusion Bodies Combined α thalassemia Deletions Detected (Total Samples n = 1025) Sens Spec Sens Spec Sens Spec All single α globin gene deletions 0.02 0.89 0.02 0.9 0.04 0.86 245 α thalassemia trait (SEA deletion) 0.69 0.96 0.96 0.99 0.97 0.98 72 Hb H disease (SEA deletion) 0.88 0.92 0.38 0.92 1.0 0.89 8 α thalassemia trait (non-SEA deletions) 0.68 0.92 0.0 0.92 0.68 0.9 19 Hb H disease (non-SEA deletions) 0.67 0.91 0.0 0.92 0.67 0.89 3 Total Hb Barts hydrops fetalis carriers 0.71 0.98 0.71 0.99 0.91 0.97 102
