Abstract
The 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