Blood is found in the exudate retrieved during a thoracentesis. This is most likely associated with:

When blood is present in the exudate retrieved during a thoracentesis, it is most often indicative of an underlying condition that leads to bleeding or significant inflammation. In this case, the presence of blood in the exudate is most likely associated with infection, particularly conditions such as parapneumonic effusion or empyema.

Infections can cause inflammation and damage to the pleura, leading to an effusion that is rich in protein and may contain red blood cells. Specifically, in cases of bacterial pneumonia, the infection can result in the development of exudative effusions that may be bloody due to the rupture of blood vessels in the pleural space or compromise of pleural integrity.

While cancer can also lead to bloody pleural effusions, infections are often more acute and can cause significant pleural effusion with associated blood. Pleural effusions themselves simply refer to the accumulation of fluid in the pleural space, which is not inherently indicative of blood; the underlying cause must be considered.

Tuberculosis can also cause pleural effusion and may be bloody; however, in the context of a thoracentesis showing blood, the acute presentation and prevalence of infections make them a more common cause of such findings.