Background Tdggedng receptors expressed on myeloid cells(TREM)proteins are a family of cell surface receptors expressed broadly by cells of the myeloid lineage.The aim of this study was to investigate the clinical significance of soluble TREM-1(sTREM-1)in pleural effusions,and to determine the effects of pneumonia on pleural sTREM-1 concentrations.Methods PleuraI fluid was collected from 109 patients who presented to the respiratory institute (35 with malignant pleural effusion,31 with tuberculous pleural effusion,21 with bacteriaI pleural effusion,and 22 with transudate).The concentrations of sTREM-1,tumor necrosis factor-o(TNF-α)and interleukin-1β(IL-1β)were determined jn effusion and serum samples by enzyme Iinked immunosorbent assay(ELISA).Results The concentrations of sTREM-1 in bacterial pleural effusion were significantly higher than those in malignant.tuberculous,and transudative groups(all P＜0.001).An sTREM-1 cutoff value of 768.1 ng/L had a sensitivity of 86%and a specificity of 93%.Pleural sTREM-1 Ievels were positively correlated with Ievels of TNF-α and IL-1β.Patients with complicating bacterial pneumonia did not have elevated concentration of STREM-1 jn pleural effusion when compared with patients without pneumonia.Conclusions Determination of pleural sTREM-1 may improve the ability of clinicians to differentiate pleural effusion patients of bacterial origin from those with other etiologies.The occurrence of bacterial pneumonia did not affect pleural sTREM-1 concentrations.