Decreased heat-labile opsonic activity and complement levels associated with evidence of C3 breakdown products in infected pleural effusions

ured simultaneously in serum and pleural fluid of pa-tients with transudates, infectious exudates (with positive or negative bacterial culture) and neoplastic exudates, using two different complement-dependent phagocytic tests: the killing of Staphylococcus aureus Wood 46 variant strain (K50 opsonic titers) and the assessment of ingestion rate of endotoxin-coated paraf-fin particles (Oil Red 0 uptake test). K50 opsonic titers were lower in culture-positive pleural effusions as com-pared to culture-negative (P < 0.002) or neoplastic ef-fusions (P < 0.002). These results were corroborated by the Oil Red 0 uptake test. The data obtained with the two assays showed a significant correlation (P < 0.001). The hemolytic activity ofcomplement (CH50) as well as the levels ofC3 breakdown product, C3d, were meas-ured in the same sera and pleural fluid samples and in an additional group ofpatients with pleural effusions of the same etiology. Effusions with positive cultures showed lower CH50 values (P < 0.01) and higher C3d values (P < 0.05) when compared to culture-negative pleural fluids. Finally, evidence for immune complexes in pleural effusions and sera was looked for by deter-mination ofClq binding activity. Levels were higher in culture-positive effusions when compared to culture-negative fluids (P = 0.005). K50 opsonic titers showed a positive correlation with CH50 values (P < 0.001) for all fluids tested. Similarly Clq binding activity correlated with C3d levels in ef