Objective To establish and assess the new method of APTT assay based on the combinations of Mg2+and Ca2+for lupus anticoagulants(LA)measurements.Methods This prospective study included 309 trisodium citrate anticoagulated plasma samples from 244 random patients and 65 patients with different autoimmune diseases(AID)to establish and assess the method of LA measurement, respectively.Final concentrations of 0,2.0, 4.0, 8.0,16.0 mmol/L Mg2+were added into 25 mmol/L Ca2+solution, and Actin reagent was used to measured plasma APTT of 94 patients.The applied concentration of Mg2+-Ca2+solution was confirmed through the special and significant alteration of APTT from LA-positive and -negative plasma observed in the presence of Mg 2+(test solution).Based on Actin reagent use,the test solution and 25 mmol/L Ca2+solution were applied to measure APTT of patients and normal individuals, respectively, and the ratio of Mg2+-Ca2+-APTT to Ca2+-APTT(Mg2+-Ca2+-APTT indices)and normalized Mg2+-Ca2+-APTT indices(NAR)were calculated, respectively.Mixed plasma NAR was measured,and CV%was calculated to evaluate the repeatability and stability of Mg 2+-Ca2+-APTT method.APTT of 150 patients was measured with the test solution and Actin reagent to calculate Mg 2+-Ca2+-APTT indices, and normalized LA ratio was determined with dRVVT method.The applicability of Mg2+-Ca2+-APTT assay was assessed through comparisons of the results from the two methods.Finally, NAR and NLR of 65 patients with AID(including 26 SLE patients)were measured with Mg2+-Ca2+-APTT assay and dRVVT method, respectively, and ROC curve was also used to assess the efficacy of the two methods for LA measurements.Results In all LA-negative plasma,APTT increased from 28.1 ±4.5 s to 61.2 ±7.9 s in normal APTT group,47.2 ±8.9 s to 97.5 ±10.3 s in increased APTT group,and 27.6 ± 5.1 s to 61.2 ±7.9 s in ACA-positive group when Mg2+increased from 0 to 8 mmol/L in Mg2+-Ca2+solution(F=34.12, 38.9 and 28.35,P<0.01).Following increased Mg2+concentration, APTT shortened from 0 to 4.0 mmol/L, but simultaneously prolonged from 4.0 to 16.0 mmol/L in LA-positive plasma with prolonged or normal APTT(F=31.55 and 39.51, P<0.01), and APTT was significantly higher in 8.0 mmol/L than that in 4.0 mmol/L(P<0.001).The test concentration of Mg 2+/Ca2+solution was 4.0 mmol/L.The within, inter-day CV% of NAR was 1.39%,2.30%, and 3.44%, respectively. According to the judging criteria of <0.966 and >1.034 of Mg2+/Ca2+indices, there was 141 patients with increased indices and NLR <1.20, and 9 patients with decreased ones and NLR≥1.20 in all 150 patients.The area under ROC curve of NAR and NLR for LA detection was 0.913(95%CI:0.848-0.978) and 0.892(95%CI:0.817-0.966), respectively, and the cut-off value was 0.87 and 1.13, respectively. The sensitivity and specificity of NAR(85% and 77%)was higher than that of NLR(81% and 74%), respectively.The accordant rate of positive,negative,and total results between NAR and NLR was 94.4%, 98.5%,and 98%,respectively.Conclusion The method of APTT assay based on Mg2+combining Ca2+for LA measurements is feasible,and can be used to detect plasma LA of patients.