Red cell parameters, HbA2 levels, and the best discrimination index in beta thalassaemia trait in Sri Lanka

Abstract

Introduction: Beta thalassaemia trait (BTT), also known as thalassaemia carrier state, leads to hypochromic microcytic indices. Identifying asymptomatic carriers on full blood count will help medical personnel to direct them for confirmatory tests such as High Performance Liquid Chromatography (HPLC) and prevent birth of babies with thalassaemia major by discouraging intermarriage between two individuals with BTT. Objectives: To describe red cell parameters and HbA2 levels of individuals with BTT in Sri Lanka and determine the best discrimination index for screening of BTT. Methods: BTT confirmed by HPLC (HbA2 ≥3.5%) during a two year period at the Medical Research Institute, Colombo, Sri Lanka were selected and full blood count (FBC) reports were analysed. Haemoglobin (Hb), RBC parameters; RBC count, MCV, MCH, MCHC, and RDW and HBA2 of the study population was analysed and the mean and the standard deviation (SD) was calculated for each parameter. These parameters were then analysed using Mentzer, Srivastava, England and Fraser, Shine and Lal, Green and King and Thal Index and sensitivity of each index for detecting BTT was calculated to ascertain which index identified BTT with maximum sensitivity. Results: 157 BTT cases without concurrent iron deficiency and 206 BTT without serum ferritin results were analysed. In the BTT group without iron deficiency, Hb was 8.3-11.1 g/dl; mean 9.7 g/dl, RBC count 4.1-5.7; mean 4.9. MCV 59.2-68.8 fl; mean 64 fl. MCH 18.1-21.9 pg; mean 20 pg. MCHC 29.4-32.6 g/dl; mean 31 g/dl. RDW 12.2-23% mean 17.6%. HBA2 level for BTT in Sri Lanka was 4.8% (SD 0.5). Sensitivity for each index was, Mentzer Index 57.1%; England and Fraser Index 25.3%; Srivastava Index 44.2%; Green and King Index 61.5% Thal Index 73.3%; Shine and Lal Index 98.7% and New index was 92.3%. Conclusions: All RBC parameters except MCHC were outside the normal ranges. A mean MCV of 64 fl (SD 4.8) and MCH of 20 (SD 1.9) best denoted BTT on FBC. The lower limit of HbA2 (4.3%) in BTT in Sri Lanka is slightly higher than the currently used diagnostic value of ≥ 3.5%. Shine and Lal Index with a sensitivity of 98.7% was the most reliable index to identify BTT in Sri Lanka. Vigilant application of above findings on FBC by first contact doctors will help to identify more thalassaemia carriers.