Utility of pleural fluid predicitve marker panels in metastatic lung and breast adenocarcinoma

<bold>Introduction:</bold> In metastatic lung and breast adenocarcinoma, pleural effusion cytology may be diagnostic but a panel of predictive markers (PMs) are needed to plan systemic anticancer therapy (SACT). Incomplete PM data prevents optimal therapy. We performed a multicentre retrospective cohort study to assess the success rate of complete PM panels, baseline predictors of complete panels, reporting times and SACT utilisation. <bold>Methods:</bold> Data were retrieved for patients with lung/breast adenocarcinoma on pleural cytology from 4 UK centres (2016-2021). PM success was defined as completion of all markers required by international guidelines. Individual marker yield & reporting time were recorded. Associations with baseline clinico-radiological features, including CT & effusion size were evaluated. PM success, individual marker yield & effusion PM marker-directed SACT rate were compared between 2016-2018 & 2018-2021. <bold>Results:</bold> 327 patients were studied. PM success rate was only 20.2%, see figure 1 for details. No baseline features were associated with PM success. PD-L1 ROS-1 & ER success rate and effusion PM marker-directed SACT rate increased over time (25% v 57%, p=0.002). Median reporting time was 19(9-24) days, overlapping with time to SACT (31(4-45) days). <bold>Conclusions:</bold> Pleural effusion PM panel results are frequently incomplete and reporting often takes weeks. Direct-to-biopsy stratification may be appropriate in some patients. <fig><object-id>erj;64/suppl_68/PA3359/F1</object-id><object-id>F1</object-id><object-id>F1</object-id><graphic></graphic></fig>