Relationship Between Pleural Fluid and Serum Cholesterol Levels

Since the criteria of Light andcolleagues for differentiating transudates and exudates were described, other tests, including the pleural fluid (PF) cholesterol test, havebeen proposed for the same purpose. However, the factors influencing PFcholesterol levels have not been clearly delineated. To analyze the relationships among totalcholesterol (CHOL), low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, and triglycerides (TRIG) inserum (S) and PF. PF and S from 99patients (transudates, 13 patients; exudates, 86 patients) wereanalyzed for CHOL, HDL, LDL, TRIG, apolipoprotein AI, apolipoprotein B, and protein. The relationship between the PF and S level for each ofthese measurements was analyzed with linear regression and multipleregression using the ratio of PF to S protein for that measurement as asecond independent variable. This studydemonstrated that CHOL levels in PF are related to S cholesterollevels and to the permeability of the pleura(r = 0.88; p < 0.001). However, the percentage of, CHOL associated with LDL and HDL (56%) in the PF was much lower thanthat associated with LDL and HDL in S (93%), suggesting thatlipoproteins are modified once they enter the pleural space. The PFTRIG was not closely related to its S level or to the PF/S proteinratio (r = 0.49). PFcholesterol levels can be closely predicted from the S cholesterollevels and the permeability of the pleura, as reflected by the ratio of, PF protein to S protein. Therefore, the CHOL ratio should not provideadditional information to that provided by the protein ratio whentrying to differentiate transudates from exudates. PF lipoproteins (LDLand HDL) undergo metabolic alterations once they enter the pleuralspace. PF TRIG levels are not closely related to S levels or to thepermeability of the pleura.