High levels of the pleural fluid macrophage ratio and mesothelial cell ratio associated with malignant pleural effusion diagnosis: Observational study

The pleural fluid macrophage ratio and mesothelial cell ratio are generally of limited diagnostic value. However, elevated ratios in some patients can complicate interpretation for physicians. This study evaluated the diagnostic utility of the pleural fluid macrophage ratio and mesothelial cell ratio across patients with various types of exudative pleural effusion. Data were retrospectively collected from 201 patients with tuberculous pleurisy, 243 with pleural infection, 432 with malignant pleural effusion, and 213 with other diseases at Fukujuji Hospital from January 2012 to June 2024. Comparisons of the pleural fluid macrophage ratio and mesothelial cell ratio were made among the 4 disease groups. Patients with malignant pleural effusion had a higher pleural fluid macrophage ratio (median 12.0%) than did those with tuberculous pleurisy (median 5.0%, P < .001), pleural infection (median 4.0%, P < .001), and other diseases (median 6.8%, P < .001). The pleural macrophage ratio was lower in pleural infection than in tuberculous pleurisy (P = .010) or other diseases (P < .001). The pleural fluid mesothelial cell ratio was elevated in patients with malignant pleural effusion, although the median pleural fluid mesothelial cell ratio was 0% across all disease groups. Among patients with a pleural fluid macrophage ratio of ≥ 30% and a pleural mesothelial cell ratio of ≥ 10%, 82 patients (71.9%) and 22 patients (77.3%), respectively, were diagnosed with malignant pleural effusion. Extremely high levels of the pleural fluid macrophage ratio (≥30%) and mesothelial cell ratio (≥10%) suggest a diagnosis of malignant pleural effusion.