Background and objectiveCancer ratio (CR), which is defined as serum lactate dehydrogenase (LDH) to pleural fluid adenosine deaminase (ADA) ratio, has been reported to be a useful diagnostic marker for malignant pleural effusion (MPE). Whether its diagnostic accuracy is affected by age remains unknown. This study aimed to investigate the effects of age on the diagnostic accuracy of CR.MethodsThe participants in this study were from a prospective cohort (SIMPLE cohort, n = 199) and a retrospective cohort (BUFF cohort, n = 158). All participants were patients with undiagnosed pleural effusion (PE). We used receiver operating characteristic (ROC) curves to evaluate the diagnostic accuracy of CR. The effect of age on the diagnostic accuracy of CR was investigated by adjusting the upper limit of age for participant enrolment.ResultsEighty-eight MPE patients were verified in the SIMPLE cohort, and thirty-five MPE patients were verified in the BUFF cohort. The AUCs of CR in the SIMPLE and BUFF cohorts were 0.60 (95% CI: 0.52–0.68) and 0.63 (95% CI: 0.54–0.71), respectively. In both cohorts, the AUCs of CR decreased with the advancement of age.ConclusionAge can affect the diagnostic accuracy of CR for MPE. CR has limited diagnostic value in older patients.Key messageCancer ratio is a promising diagnostic marker for malignant pleural effusion.This study revealed that its diagnostic accuracy decreased in older patients.Its diagnostic accuracy is overestimated by previous studies using tuberculosis and pneumonia patients as controls.