The pathogenesis of low glucose, low pH malignant effusions.

Possible mechanisms to explain the finding of a low pH, low glucose, malignant pleural effusion include: use of glucose and acid production by pleural fluid constituents including leukocytes and free malignant cells; pleural membrane metabolism, especially by malignant cells; abnormal transfer of glucose, carbon dioxide, and hydrogen ion across a diseased pleural membrane. To determine the pathogenesis of low glucose, low pH effusions, we performed incubation and glucose and gas transport studies in 5 patients with malignant effusions, 3 with a low pH (less than 7.30) and 2 with a pH greater than 7.30 (control patients). After 24 h of incubation, there was no significant difference in the metabolic activity of pleural fluid between low pH fluids and control fluids. Transport studies confirmed impaired glucose transfer both into and out of the pleural space and impaired efflux of CO2 from the pleural space in patients with low pH effusions, whereas control patients demonstrated free transfer across the pleural membrane. It appears that an abnormal pleural membrane (tumor or fibrosis), rather than increased acid production, results in a low glucose concentration from impaired glucose transfer from blood to pleural fluid and a low pH from impaired hydrogen ion efflux in some malignant effusions.