Introduction and objective:
Thoracocentesis is an invasive procedure routinely performed in the diagnosis of causes for the presence of pathological fluid in the pleural cavity. In many patients, a computed tomography scanning (CT) is also performed to diagnose the cause of the presence of fluid in the pleural cavity. The diagnostic value of CT is particularly high in situations in which performing thoracocenthesis could be associated with an increased risk of complications. The aim other study was to assess the relationship between the objective radiological features and the results of laboratory tests of fluid collected by thoracocenthesis in patients with pneumo-nias (n=18) and lung cancer (n=35).

Material and methods:
The examined group consisted of the patients with pneumonia (n=18) and lung cancer (n=35) which resulted in the presence of fluid in the pleural cavity. In the patients thoracocentesis, CT lung scanning was also performed, according to the medical indications. Three scans with the greatest amount of fluid were identified, and the mean density of the fluid expressed in Hounsfield units was calculated within the area. These calculations were compared with the results of laboratory fluid tests.

Results:
The maximum number of Hounsfield units (HU) was significantly lower in the group of lung cancer patients, compared to those diagnosed with pneumonia (74.3% sensi-tivity and 55.6% specificity). The pH of pleural fluid was significantly lower in patients with lung cancer, compared to those with pneumonia (74.3% sensitivity and 66.7% specificity).

Conclusions:
According to the results, radiological differentiation of pneumonia and lung cancer resulting in pleural effusion, to some extent is possible; however, the needle is still needed.