Parapneumonic effusion and empyema

What it is

A parapneumonic effusion is a pleural effusion from pneumonia, and it comes in three steps. In an uncomplicated parapneumonic effusion the etiology is movement of fluid from the pneumonia into the visceral pleura, giving a sterile exudate in the pleural space. In a complicated parapneumonic effusion there is persistent bacterial invasion into the pleural fluid. In empyema there is bacterial colonization with purulent fluid. Empyemas are exudative effusions with a low glucose concentration, due to the high metabolic activity of leukocytes and bacteria within the pleural fluid.

How it is diagnosed

The pleural fluid analysis separates the three. An uncomplicated parapneumonic effusion has a pH of 7.2 or above, glucose of 60 mg/dL or above, and WBC of 50,000/mm3 or below, with glucose decreased to normal and an LDH ratio above 0.6. A complicated parapneumonic effusion has a pH below 7.2, glucose below 60 mg/dL, and WBC above 50,000/mm3. Empyema has a pH below 7.20, low glucose and an LDH ratio above 0.6, with purulent, neutrophil-predominant fluid.

Pleural fluid Gram stain and culture are negative in the uncomplicated effusion. In the complicated effusion the Gram stain is negative and the culture is usually negative, and this is typically a false negative due to the low bacterial count. In empyema the Gram stain is positive and the culture is usually positive.

All of these are exudates by the Light criteria, which define an exudate as at least one of three: the first, a pleural fluid protein to serum protein ratio above 0.5; the second, a pleural fluid lactate dehydrogenase to serum LDH ratio above 0.6; the third, a pleural fluid LDH above 2/3 of the upper limit of normal for serum LDH. A pleural fluid glucose below 60 mg/dL is usually due to rheumatoid pleurisy, complicated parapneumonic effusion or empyema, malignant effusion, tuberculous pleurisy, lupus pleuritis, or esophageal rupture, and a pleural glucose below 30 mg/dL in particular suggests an empyema or rheumatic effusion.