Abstract: Objective To investigate the clinical significance of droplet digital polymerase chain reaction (ddPCR) in detecting epidermal growth factor receptor (EGFR) mutation in circulating tumor DNA (ctDNA) of lung adenocarcinoma. 
Methods The ddPCR was used to analyze EGFR mutation (EGFR-L858R and EGFR-19Del mutations) in plasma ctDNA in 102 patients with lung adenocarcinoma. The relationship between ctDNA and clinical parameters was analyzed. The ROS curves were drawn to evaluate the predictive effect of ctDNA on prognosis and metastasis of patients. 
Results The area under the ROC curve of ddPCR in detecting EGFR mutation in ctRNA for evaluation of EGFR mutation was 0.964, and the sensitivity and specificity were 40.2% and 88.6% respectively. Compared with early-stage lung adenocarcinoma, advanced-stage lung adenocarcinoma had higher AUC (0.837 vs. 0.498) and sensitivity (75.2% vs. 10.2%). Tumor metastasis and fractional abundance of ctDNA showed a strong correlation (r=0.51, P＜0.001). There was a significantly shorter progression-free survival (PFS) and overall survival (OS) in the ctDNA-positive group than in the ctDNA-negative group (13.8 months vs. 42.2 months, P＜0.05, 34.4 months vs. 66.8 months, P＜0.05). PFS of ctDNA-positive group treated with human epidermal growth factor receptor tyrosine kinase inhibitor was shorter than that of ctDNA-negative group (12.0 months vs. 24.0 months, P＜0.05). In addition, OS was shorter in ctDNA-positive patients than in ctDNA-negative patients (34.4 months vs. 66.8 months, P＜0.05). 
Conclusion DdPCR can be used to detect EGFR mutation in ctDNA of patients with advanced lung adenocarcinoma. EGFR mutation-positive lung adenocarcinoma in ctDNA indicates that patients may have distant metastasis and poor prognosis.

Key words: adenocarcinoma of lung, circulating tumor DNA, polymerase chain reaction