BACKGROUND: In the process of implementing the nucleic acid amplification tests (NAT) for the blood screening, it was needed to change plain tube to EDTA tube for the sampling. Because the sample is taken from the CPDA-1 anticoagulated whole blood, the EDTA of tube could be mixed with the CPDA-1. So, we studied the effect of the mixing of two anticoagulants on the NAT. METHODS: Using HIV-1 and HCV RNA standards, we made the qualitative and quantitative test panels for the EDTA anticoagulant and the EDTA/CPDA-1 anticoagulant containing blood. The reverse transcription-polymerase chain reaction of Roche and transcription-mediated amplification of Chiron were used for the RNA qualitative and quantitative test. RESULTS: On the qualitative HIV-1 and HCV RNA tests for the EDTA, CPDA-1 alone and the CPDA-1/EDTA mixture, false negative and false positive reactions were not observed. On quantitative test, viral loads were not different statistically. CONCLUSIONS: Since there were no statistically significant differences between CPDA-1 alone and EDTA/CPDA-1 mixture in both qualitative and quantitative tests for HIV-1 and HCV RNA, it was concluded that mixing of anticoagulants, EDTA and CPDA-1, would not cause an significant effect on the NAT for the donated blood.