What is a characteristic feature of empyema seen on thoracentesis?

The characteristic feature of empyema on thoracentesis is the presence of bacterial organisms on Gram stain. Empyema is defined as the accumulation of pus in the pleural space, typically resulting from infection (such as pneumonia or a lung abscess). When fluid from the pleural space is analyzed, a Gram stain can be performed to identify the presence of bacteria. A positive Gram stain indicates that there are bacteria present, which is a direct reflection of the infectious nature of empyema.

In contrast, empyema is associated with fluid that is often thick and turbid due to the high cellularity and presence of pus. The other options do not correctly represent the features seen in empyema: the pH is typically low (often < 7.2) due to the presence of lactic acid from bacterial metabolism, pleural fluid is not clear but rather cloudy or opaque because of the presence of white blood cells and organisms, and the glucose content in empyema fluid can be low due to consumption by the bacteria. Thus, the presence of bacteria seen on Gram stain is a key diagnostic feature of empyema.