Osimertnib is still widely used in the treatment of NSCLC patients who have previously received erlotinib, gefitinib or afatinib and have developed resistance to these drugs mediated by the T790M mutation in exon 20 of EGFR gene. We assessed the results of T790M mutation testing in liquid biopsy by Entrogen test and real-time PCR technique in routine clinical practice. Analysis was conducted in 73 plasma samples from 41 patients with locally advanced or metastatic lung adenocarcinoma treated with first- or second-generation of EGFR TKIs. We detected T790M mutation in 18 patients (43.9% of patients, 24.6% positive tests in 73 samples). The incidence of T790M mutation in liquid biopsy was significantly higher in patients with T3-T4 tumors compared to patients with T0-T2 tumors (p = 0.0368, χ2 = 4.36). Median PFS at the time of progression according to RECIST was significantly (p = 0.0444) higher in patients with T790M mutation than in patients without this mutation (22.5 vs. 15 months). Our results confirmed that T790M mutation is more often detected in patients with a large tumor spreading in the chest and with the long duration of response to first- or second generation of EGFR TKIs. The low sensitivity of the real-time PCR technique in T790M mutation detection could be partially compensated by repeating the tests.