Introduction: _-thalassemia carrier screening in Sri Lanka primarily relies on High-Performance Liquid Chromatography (HPLC), with elevated HbA₂ levels considered the diagnostic hallmark. However, variants affecting _-globin synthesis can reduce HbA₂ levels and mask _-thalassemia carrier status. With increasing use of genomic testing, such hidden interactions are being recognized and have important implications for paediatric and population-level screening. Case presentation: We report a 6-year-old female who underwent posthumous whole exome sequencing (WES) following sudden death due to suspected familial cardiomyopathy , with a history of sudden death in a younger sibling. WES identified a homozygous likely pathogenic splice-site variant in NRAP, consistent with autosomal recessive dilated cardiomyopathy ,explaining the primary clinical phenotype. Incidental carrier findings revealed a heterozygous pathogenic HBB splice variant (c.92+5G>C), consistent with _-thalassemia trait. Importantly , the proband also carried a heterozygous HBD missense variant (c.82G>T; Hb A2- Yialousa), known to reduce _-globin production. Coinheritance of these variants is associated with normal or low HbA₂ levels, potentially resulting in false-negative _-thalassemia screening on routine HPLC testing. Conclusions: This case highlights an important limitation of HbA₂-based _-thalassemia screening, where coinherited HBD variants can mask carrier status. Molecular testing should be considered in individuals with discordant laboratory findings or relevant family history . Increased awareness of such genetic interactions is essential to improve the accuracy of thalassemia screening programs in high-prevalence settings.