COBAS ver2. Result: Fifteen patients were enrolled. In ten patients whose EGFR mutation was detected in liquid biopsy, the original EGFR mutation (exon 19 del or L858R) was detected in all patients. The detection rate of T790M was lower than that of the original EGFR mutation in liquid biopsy compared to that in tissue re-biopsy. The detection of T790M in serum exhibited a higher specificity (67%) and positive predictive value (50%) than that in plasma (50% and 40%, respectively). The detection sensitivity was similar in plasma and serum. Nine patients were treated with osimertinib. The RR was 77.8% and DCR was 100%. One patient who presented a response was positive for T790M in liquid biopsy (both plasma and serum) and negative in tissue re-biopsy. Conclusion: We suggest serum samples to be more useful than plasma samples for determining the effectiveness of osimertinib against relapse tumor sites because they were more reliable in the detection of T790M mutation at the relapse tumor tissue sites. Repeated tests with different samples and different methods may improve accuracy of T790M detection and will lead to the maximum benefit for the patient.