Screening for alpha (α) thalassaemia trait (TT) is challenging especially in the presence of iron deficiency (ID). Red blood cell size factor (RSf) is a parameter introduced by Beckman Coulter capable of detecting acute and chronic changes to cellular haemoglobin status and iron supply. The research aimed to investigate the clinical usefulness of RSf as screening parameter for α TT and the effect of concomitant ID to RSf mean values (m.v) among respondents with α TT. Methods: A cross-sectional retrospective laboratory analysis involved 55 respondents’ data selected from January 2014 to December 2015 in Pathology Department, Hospital Tuanku Ja’afar Seremban, Negeri Sembilan. The significant difference at p