How is an empyema diagnosed?
Diagnosis is made when pleural fluid is grossly purulent, when pleural fluid has a positive Gram stain or culture, or when the white blood cell count is greater than 5 × 10 9 cells per liter. Anaerobic bacteria, usually streptococci or gram-negative rods, are responsible in about 75% of cases. Chest radiographic findings of empyema are similar to findings of other loculated pleural fluid collections. CT findings may include increased attenuation of the fluid caused by protein, cells, or hemorrhage; septations and loculation; foci of very low attenuation gas; thickened enhancing pleural surfaces (the “split-pleura” sign); or increased attenuation of the extrapleural fat ( Figure 21-4 ). MRI findings may include increased signal intensity of the fluid on T1-weighted images, decreased signal intensity of the fluid on T2-weighted images, very low signal intensity foci of gas, internal septations, and thickened enhancing pleural surfaces.