The EarlyCDT-Lung test is a high specificity blood-based autoantibody biomarker that could contribute to predicting lung cancer risk. Here we report on the results of a phase IV biomarker evaluation of whether using the EarlyCDT-Lung test and any subsequent CT scanning to identify those at high risk of lung cancer reduces the incidence of patients with stage III/IV/Unspecified lung cancer at diagnosis, compared with the standard clinical practice at the time the study began.ECLS was a randomised controlled trial of 12 208 participants at risk of developing lung cancer in Scotland. The intervention arm received the EarlyCDT-Lung test and, if test positive, low-dose CT scanning six-monthly for up to 2 years. EarlyCDT-Lung test negative and control arm participants received standard clinical care. Outcomes were assessed at 2 years post-randomisation using validated data on cancer occurrence, cancer staging, mortality and comorbidities.At 2 years, 127 lung cancers were detected in the study population (1.0%).In the intervention arm, 33/56 (58.9%) lung cancers were diagnosed at stage III/IV compared to 52/71 (73.2%) in the control arm. The hazard ratio for stage III/IV presentation was 0.64 (95% confidence interval 0.41, 0.99). There were non-significant differences in lung cancer and all-cause mortality after 2 years.ECLS compared EarlyCDT-Lung plus CT screening to standard clinical care (symptomatic presentation), and was not designed to assess the incremental contribution of the EarlyCDT-Lung test. The observation of a stage-shift towards earlier-stage lung cancer diagnosis merits further investigations to evaluate whether the EarlyCDT-Lung test adds anything to the emerging standard of LDCT.