Table 2: Different stages in the evolution of an infected pleural effusion with associated pathological changes and pleural fluid findings.

PhasePathologyPleural fluid findings

ExudativeIncreased permeability of vascular and visceral pleural membranes VEGFNonviscous
Free flowing
Readily drained
Pleural fluid Cx negative

FibrinopurulentFibrin deposition on visceral pleura Locules formation IL-8, TNF-αpH > 7.20
Glucose within normal ranges
LDH < 3 times ULN
More viscous
Pleural fluid cx positive
Typical “complicated” effusion

OrganizingFibroblast entry Pleural peel
Thick pus
Very viscous
pH < 7.20
Glucose < 40
LDH > 3 times ULN

LDH: lactate dehydrogenase.
ULN: upper limits of normal.
VEGF: vascular endothelial growth factor.
IL-8: interleukin 8.
TNF-α: tumor necrosis factor-alpha.
TGF-β: transforming growth factor-beta.