Neopterin is derived from guanosine-triphosphate, produced by stimulated macrophages under the influence of gamma interferon of lymphocyte origin. It has been suggested as an excellent marker for activation of the monocyte/macrophage axis in some clinical situations. We evaluated its concentration in the pleural effusions of 25 individuals (10 tuberculous and 15 neoplastic) as well as in the blood of 22 of them (8 tuberculous and 14 neoplastic), comparing these levels with those of a control group in 99 normal individuals. The concentration of neopterin was determined by radioimmunologic assay. This showed a significant increase (p less than 0.001) of neopterin levels in the tuberculous pleural fluid, compared to the neoplastic group (42 +/- 23/17 +/- 9 nmol/L). In the blood, values were nearly identical to the pleural fluid (41.3 +/- 25/15.8 +/- 6.9 nmol/L), although with significant differences between them and in relation to the control group (p less than 0.001), which had a normal serum value (5.11 +/- 1.92 nmol/L). We emphasize the influence of the neopterin levels in the pleural fluid on the diagnosis of causes of pleurisy and its importance as a marker of immunologic cellular activity.

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