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Objective To evaluate the value of HbA2 level determined by capillary electrophoresis (Hb-CE)in screening and diagnosis of thalassemia.Methods HbA2 level of 249 thalassaemia carriers and 142 healthy controls confirmed by molecular biological detection were determined by Hb-CE method.The thalassaemia carrier subjects were divided into different groups and subgroups according to their results of gene detection.The sensitivity,specificity,accuracy,positive predictive value and negative predictive value for the diagnosis ofα-thalassemia,β-thalassaemia,α,β-thalassaemia were calculated under different HbA2 cut-off value.Results Mean value of HbA2 in healthy controls was (3.03±0.27)%.Mean values of HbA2 inα-thalassemia group and its subgroups of silentα-thalassemia,standardα-thalassemia and hemoglobin H disease were (2.38± 0.55)%,(2.61±0.46)%,(2.47 ± 0.32)% and (1.07 ± 0.17)%,respectively.Mean values of HbA2 inβ-thalassaemia group and itsβ0 subgroup,β+ subgroup were (5.65±0.47)%,(5.71±0.48)% and (5.56±0.43)%.Mean value of HbA2 in compoundαandβ-thalassaemia group was (5.7±0.82)%.Compared with healthy controls,HbA2 level inα-thalassemia group,silentα-thalassemia subgroup,standardα-thalassemia subgroup and hemoglobin H disease group decreased signifi-cantly (t values of 11.73,5.02,12.91 and 33.46,respectively,P<0.01).HbA2 level in hemaglobin H disease was signifi-cantly lower than silent and standardα-thalassemia subgroups (t values of 15.62 and 21.31,respectively,P<0.01),but there were no differences in HbA2 level between silent and standardα-thalassemia subgroups (t=1.50,P>0.05).HbA2 level inβ-thalassaemia group,β0 subgroup,β+ subgroup and compoundαandβ-thalassaemia group increased significantly (t values of 55.12,44.33,38.94 and 9.10,respectively,P<0.01),but there were no differences in HbA2 level betweenβ0 andβ+ subgroups (t=1.79,P>0.05).Of 249 thalassemia carriers,all 124β-thalassaemia carriers were distinguished with ele-vated HbA2 level (>3.5%)determined by Hb-CE and only 57 were distinguished from 117α-thalassemia carriers by Hb-CE.Under the cut-off value of 2.5%,the sensitivity,specificity,positive predictive value,negative predictive value and accu-racy for the diagnosis ofα-thalassemia were 48.72%,97.18%,93.44%,69.70%,75.29%,respectively.Under the cut-off value of 3.5%,they were 100.00%,98.59%,98.41%,100%,and 99.25% for the diagnosis ofβ-thalassaemia,respectively. The analysis of ROC curve showed that the optimal HbA2 cut-off values for diagnosis ofα,β-thalassaemia by capillary elec-trophoresis were 2.8% and 3.7%,respectively.Conclusion When no abnormal bands,the elevated HbA2 (>3.7% in this study)determined by Hb-CE could be used as a marker forβ-thalassaemia diagnosis,but theβ-thalassaemia co-existingα-thalassemia could not be differentiated fromβ-thalassaemia diagnosis.Decreased HbA2 level (<1.5% in this study)and HbH band could be used for the diagnosis of hemoglobin H disease.Only HbA2 determination by Hb-CE has no clinical sig-nificance for the screen and diagnosis ofα-thalassemia.