Screening For Thalassemia Carriers In Populations With High Rate Of Iron Deficiency: Revisiting The Applicability Of Mentzer Index and The Effect Of Iron Deficiency On HbA2 Level

Iron deficiency (ID) and beta thalassemia carrier state (Tm) are the two most common causes of microcytosis in pediatric population. Differentiating between these conditions has therapeutic significance as well as important implications in thalassemia carrier screening. Since Menzter first reported his observation on the applicability of “Mentzer Index” in differentiating Tm from IDA with a reasonable reliability, several other formulas have been developed and tested. However, one major concern, that is the high probability of the concurrence of the two conditions in countries where both ID and Tm are common, limits their use. It is not possible to separate ID from Tm with these formulas when considerable portion of patients have both conditions. This study was done to investigate the function of Mentzer Index in patients with concurrent ID and Tm. In addition we investigated the effect of ID on HbA2 level in Tm subjects and ID only patients as several previous studies have reported conflicting results on the effect of ID on the HbA2 levels.

We retrospectively analyzed the CBC, iron indices and HPLC results of pediatric patients (1-18 years) who were referred to a tertiary laboratory in Iran for over a 10 year period and had a diagnosis of ID only, Tm only, or ID+Tm. Only those who had complete CBC, RDW, ferritin and HPLC were included. Diagnosis of Tm was based on HbA2 >3.5%. ID only patients were diagnosed based on HbA2 <3.5%, ferritin <20 μg/dL and resolution of microcytosis following iron supplementation. Patients with ID+Tm were diagnosed if ferritin was < 20 μg/dL and HbA2 was > 3.5%. We defined modified Mentzer Index (mMI) as (MCV/RBC)-13. 45 subjects (17 with IDA) had confirmed diagnosis of Tm with genetic testing, all with HbA2> 3.5.

562 Patients were qualified (ID only: 333, Tm only: 131, ID+Tm: 98 patients, 286 females). mMI was < 0 in 127/131 (96%) of Tm only, and > 0 in 243/333 (72%) in ID only patients. Interestingly, however, in patients with ID+Tm, mMI was < 0 in 94/98 (95%). Further analysis (Spearman correlation) indicated that mMI was significantly correlated (inversely) with HbA2 level in Tm patients with or without ID [P<0.001], and in ID only patients [P=0.017]. Ferritin level was not correlated with mMI in Tm patients with or without ID [P=0.581], or in ID only patients [P=0.718]. In addition, HbA2 was correlated with ferritin in ID only patients [p=0.007], but not in Tm patients with or without ID [p=0.173], including in those Tm subjects with genetic testing.

It is well recognized that individuals with Tm have higher RBC counts compared to ID patients. The underlying molecular pathophysiology of this, however, is not well defined. Our study demonstrates that the number of RBCs in relation to patient’s MCV directly correlates with HbA2 (inverse relation to mMI). Also, the presence or the degree of iron deficiency does not affect mMI, so the variation in RBC numbers in relation to the degree of microcytosis is related to HbA2 level only and not ID. It appears that mMI (or Mentzer Index) separates Tm from non-Tm individuals rather than Tm from ID patients. Further studies are required to investigate the molecular basis of phenotypic variations of erythropoiesis in relation to different hemoglobins variants like HbA2. Mentzer index can be used, with reasonably high sensitivity, to initiate screening for Tm regardless of iron status, in low resource settings.

Furthermore, in our population, while ferritin level was associated with decreased HbA2 in ID only patients, this effect was not observed in individuals with Tm. We have demonstrated this in a large number of patients with ID only, Tm only, and ID+Tm, in contrast to previous studies which were done on either limited numbers or on subsets of patients. In healthy individuals, there is a relative excess of α chains compared to β chains. In the presences of ID, more reduction of α chains compared to β chains is observed which results in increased competition between β and δ chains for limited available α chains and subsequent reduction of HbA2 levels. However, in Tm patients where β chain production is also reduced, relative reduction of α chain due to ID will not be severe enough to reduce the formation of HbA2 in favor of HbA. It appears that correction of ID is not required to diagnose thalassemia carriers by HPLC or Hb electrophoresis. Of note we did not include Tm individuals with borderline/normal HbA2 (<3.5%) in this study.

No relevant conflicts of interest to declare.