Abstract Distinguishing between different causes of microcytic anemia is essential for management of anemic patients. More than 20 different RBC indices have been used to screen thalassemia trait with inconsistent results. Differentiation between iron deficiency anemia (IDA) and alpha-thalassemia appears to be more challenging than differentiation between IDA and beta-thalassemia, based on RBC indices alone. Moreover, thalassemia and IDA may coexist. A highly sensitive screening algorithm is needed to select clinical cases for alpha globin gene analysis to reduce unnecessary testing and improve pre-test probabilities. A retrospective study was performed to evaluate the reliability of 22 RBC indices (RBC, MCV, RDW, MCH, Mentzer Index, RDW Index, etc) and formulae in differentiating cases with 1-2 alpha globin gene deletions from negative cases, in a cohort of 90 individuals with confirmatory alpha globin gene analysis results. The number of patients with each alpha globin gene deletion pattern were: 37 patients with alpha-thalassemia trait, 21 silent carriers (one gene deletion), 31 negative patients and 1 patient with triplicated alpha globin gene. The mean RBC count in each group were 4.79 +/- 0.65 (trait), 4.45 +/- 0.73 (carrier), and 4.21 +/- 0.84 (negative) (million/mm). The mean MCV in each group were 71.1 +/- 4.8 (trait), 76.1 +/- 5.9 (carrier), and 74.2 +/- 9.3 (negative) (fl). The mean RDW in each group were 16.5 +/- 3.0 (trait), 17.2 +/- 3.4 (carrier), and 18.3 +/- 4.9 (negative) (%). The mean MCH in each group were 22.0 +/- 1.8 (trait), 23.8 +/- 2.3 (carrier), and 23.5 +/- 3.7 (negative) (pg). None of the RBC indices showed significant difference between carrier and negative groups, while 20 RBC indices showed significant difference (p<0.05) between trait and negative groups. The sensitivities of the RBC indices in detecting 1-2 alpha globin gene deletions based on published cutoffs ranged between 3.4% and 81.0%, while the specificities ranged between 41.9% and 100%. A novel scoring algorithm based on the combination of 12 RBC indices and formulae achieved a sensitivity of 96.6% in detecting 1-2 alpha globin gene deletions, with a negative likelihood ratio of 0.15. The negative likelihood ratio of the scoring algorithm outperformed each of the 12 RBC indices at similar levels of sensitivities (>95%) based on revised cutoffs. The algorithm is a highly sensitive tool for screening of alpha globin gene deletions, including one gene deletion. It is more effective at identifying negative cases compared to individual RBC indices. Implementation of the algorithm could efficiently reduce 10.0% (9/90) of the test volume of alpha globin gene analysis. A more stringent version of the algorithm could achieve 100% sensitivity and reduce 5.6% (5/90) of the alpha globin gene analysis test volume.
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