Cholesterol in pleural exudates depends mainly on increased capillary permeability

Pleural fluid (PF) cholesterol is a useful parameter to differentiate between pleural transudates and exudates, although the pathophysiologic mechanisms for its increase in exudates are not fully understood. We aim to elucidate the cause of this increase by analyzing the levels of cholesterol—high-density lipoproteins (HDLs), low-density lipoproteins (LDLs), apoprotein A (ApoA), and apoprotein B (ApoB)—in PF and blood as well as the number of leucocytes and red cells in the PF. We studied 259 patients with pleural effusion (57 transudates and 202 exudates). The correlations of the pleural and serum (S) levels of these parameters were analyzed, with the pleural cholesterol fractions as the dependent variables and their levels in blood and the pleural/serum protein ratio (P/S prot ratio) as the independent variables. The pleural fluid cholesterol levels (PFCHOL) correlated with their blood levels and the capillary permeability (r=0.885). No significant differences were found between the percentage of LDL, with regard to total cholesterol in the blood (concentration of cholesterol in the serum [SCHOL]), and the same percentage in the exudates, between the PF/S LDL ratio (0.46) and the PF/S CHOL ratio (0.48), or between the PF/S ApoB ratio and the PF/S LDL ratio. The percentage of PF cholesterol bound to HDL and LDL was significantly higher (91.9%) than in the blood (90%). No significant correlations were found between any of the lipids studied and the number of erythrocytes and leucocytes. In conclusion, the PFCHOL may be predicted from the SCHOL, and the capillary permeability may be reflected by the PF/S prot ratio.