From Basics to Benchmarks: Evaluating Sample Adequacy, <scp>PD</scp> ‐ <scp>L1</scp> Expression, and Molecular Profiling in Effusion Samples of Lung Adenocarcinoma

ABSTRACT Introduction Lung adenocarcinoma commonly causes malignant pleural effusion (MPE), a condition with poor prognosis and limited treatment options. Pleural effusion specimens offer a minimally invasive source for diagnosis and molecular testing. This study evaluates cytologic adequacy for molecular analysis, characterizes mutational and PD‐L1 profiles, and correlates findings with survival outcomes. Methods Over 10 years, a review of 2400 MPE identified 94 lung adenocarcinoma cases that underwent next generation sequencing (NGS). Patient demographics, cytological characteristics, molecular features, and survival outcome were integrated for analysis. Results The mean effusion volume submitted for cytological and molecular testing was 589.8 mL (range 4–2790 mL) and mean tumor cellularity was 40% (range 10%–90%). NGS identified mutations in 86/94 cases (91%). TP53 was the most common alteration (42/94, 44.6%). FDA‐approved biomarkers were detected: KRAS (28/94, 29.7%); EGFR (15/94, 15.9%); BRAF (8/94, 8.5%); ALK (3/94, 4.2%), ROS1 (2/94, 2.1%), RET fusions (1/94, 1%); and MET amplification (2/94, 2.1%). Smoking was associated with bloody effusions ( p = 0.049), frequent TP53 mutations ( p = 0.018), and fewer EGFR mutations ( p = 0.012). Patients with effusions as primary presentation was associated with higher mortality ( p = 0.002), frequent TTF‐1 positivity ( p = &lt; 0.001) and frequent BRAF mutations ( p = 0.049). TTF‐1 positivity was associated with higher PD‐L1 scores ( p = 0.035). Survival was shorter in male patients ( p = 0.045), smokers ( p = 0.025), concurrent pericardial effusions ( p = 0.027), and with TP53 mutation ( p = 0.002). Conclusions Molecular testing of effusion samples is feasible even at low fluid volumes, provided tumor cellularity is adequate. Prioritizing tumor fraction over volume ensures a higher likelihood of successful molecular analysis. Effusion‐based NGS reliably identifies actionable alterations with diagnostic and prognostic relevance. Male patients, smokers, concurrent or primary presentation with MPE, and those with TP53 mutations had poorer outcomes.