Aims The present study aims to investigate the mutation spectrum of thalassaemia, and characterise the erythrocyte indices of thalassaemia in a large cohort in Sichuan, which is a province with a high prevalence of thalassaemia in southern China.

Methods The present study was conducted from July 2017 to July 2019. A total of 42 155 women screened for thalassaemia were enrolled. The thalassaemia carriers were screened by erythrocyte analysis and haemoglobin electrophoresis. Then, the screening test results and genetic results were collected.

Results A total of 1109 individuals had thalassaemia gene defects. Among these individuals, 69.7% were α-thalassaemia (α-thal) and 28.6% were β-thalassaemia (β-thal). For α-thal defects, carriers with ^--SEA had the lowest mean corpuscular volume (MCV) and mean corpuscular haemoglobin (MCH) values. For β-thal defects, carriers with heterozygous haemoglobin E and −28 had the highest MCV and MCH values. In addition, an MCV cut-off of 80 fl and an MCH cut-off of 27 pg was able to detect 99.1% α0 thalassaemia and 99.7% β0/β+ thalassaemia; however, that criterion could result in a great number of false-negative results in α+ carriers.

Conclusion A criterion of MCV <80 fl and MCH <27 pg is recommended for detecting –SEA carriers and β0/β+ carriers, while higher cut-offs are needed to detect α+ carriers.