Techniques for primary screening for haemoglobinpathies in Sri Lanka: a comparison of single tube osmotic fragility (STOP) and dye test {DCIP) vs full blood count (FBC)

Abstract

INTRODUCTION: Techniques best suited for haemoglobinopathy screening of the Sri Lankan population need to be determined. Full blood count is used to detect p thalassaemia trait but is affected by iron deficiency. Its ability to detect haemoglobin (Hb) E is less certain. Alternative tests for screening include single tube osmotic fragility (STOP) test (for [5 thalassaemia) and a dye test (DCIP) for HbE. However there is no consensus regarding their usage in population surveys. OBJECTIVES: To decide the validity of STOP and DCIP test as screening tests for haemoglobin disorders in Sri Lanka. DESIGN, SETTING AND METHODS: Blood samples of volunteers attending an educational campaign held at the Faculty of Medicine, Ragama were tested using STOP, DCIP and FBC, and the results compared with the "gold standard" HPLC (high performance liquid chromatography) Results: 1062 individuals participated. HPLC identified 27 p traits and 3 HbE carriers. A total of 508 subjects had either MCV < 80 or MCH <27 on FBC, which included 26 of the p traits and all with HbE. The rest were presumably iron deficient. Only 56 subjects had a positive STOF test including 26 with p trait. 34 had a positive DCIP test including all three HbE carriers. One person with HbA2 of 3.5% had a normal MCV, MCH, negative STOF and negative DCIP. STOF test had a sensitivity and specificity of 96% and 97% for p trait. The DCIP had a sensitivity and specificity of 100% and for the detection of Hb E. CONCLUSIONS: Both STOF and DCIP are excellent tests for primary screening. The STOF is a superior test for screening thalassaemias than FBC in populations with high prevalence of iron deficiency.