Oncological treatment of patients with disseminated non-small cell lung cancer (NSCLC) is depending on the status of programmed death-ligand 1 (PD-L1), anaplastic lymphoma kinase (ALK) and epidermal growth factor receptor (EGFR), so called oncodrivers. These can be measured in pleural fluid, but the prevalence is uncertain. In a prospective study, the research team aim to measure PD-L1, ALK and EGFR in patients with pleural fluid cytology positive for NSCLC to report the prevalence. Also, the study will investigate if the chance of obtaining oncodriver status is depending on the volume analysed and how the lack of oncodrivers influence the following work-up.

The study is a prospective, non-randomized, cohort study of patients with pleural effusion. Participants will be recruited from patients referred to the Pleura Clinic or admitted at the ward at the Department of Respiratory Medicine, Næstved Hospital, Næstved or at the Department of Respiratory Medicine, Zealand University Hospital, Roskilde, which is the two regional centres for workup of pleural effusions. Patients will be referred from either general practice or other hospital departments.

Pleural fluids with cytology positive for NSCLC will be tested for oncodrivers (for squamous cell carcinomas (SCC): PD-L1, for adenocarcinomas (AC): PD-L1, ALK and EGFR).

Follow-up will be 8 weeks after inclusion.

Inclusion Criteria:

• Age ≥ 18 years
• Pleural effusion known or suspected of association with NSCLC (pleural fluid cytology positive for cells from NSCLC)
• Patients must be able to give informed consent

Exclusion Criteria:

• Full oncodriver status measured in any pleural fluid in current work-up
• Inability to understand written or spoken Danish.