Empyema is a collection of pus in the pleural space, the thin gap between your lungs and chest wall. Under normal conditions, this space contains only a small amount of fluid that helps your lungs expand and contract smoothly. When bacteria invade this space, typically as a complication of pneumonia, the body’s immune response produces thick, infected fluid that becomes trapped. It is a serious condition that requires prompt treatment, usually involving both antibiotics and drainage.
How Empyema Develops
The pleural space is normally sterile. Empyema most often begins when a nearby lung infection, like pneumonia, inflames the tissue lining the lungs. That inflammation makes the lining more permeable, allowing bacteria to cross into the pleural space. Once bacteria establish themselves there, the immune system floods the area with white blood cells. The resulting mix of dead cells, bacteria, and inflammatory debris is what forms pus.
Pneumonia is the most common trigger, but empyema can also develop after chest surgery, a penetrating chest injury, or the spread of an infection from elsewhere in the body. The bacteria responsible differ somewhat between adults and children. In adults, a range of common respiratory bacteria cause most cases. In children, the most frequently isolated organisms are Streptococcus pneumoniae, Staphylococcus aureus, and Haemophilus influenzae type B. Staphylococcus aureus is especially dominant in developing countries and tends to increase during hot, humid months.
The Three Stages of Empyema
Empyema progresses through three stages, first described by the American Thoracic Society roughly 60 years ago. These stages are continuous, meaning one flows into the next without a sharp dividing line, and early treatment can stop the process before it advances.
Stage 1: Exudative
In the earliest phase, fluid begins building up in the pleural space as the inflamed lung lining leaks. This fluid is thin and relatively free-flowing. At this point, it may not yet contain bacteria, and antibiotics alone can often resolve the problem. If treatment is delayed or inadequate, the infection moves to the next stage.
Stage 2: Fibrinopurulent
Bacteria now actively invade the pleural space. The body responds aggressively: white blood cells attack the bacteria, producing pus. At the same time, sticky protein strands called fibrin form web-like partitions in the fluid, dividing it into walled-off pockets (loculations). These pockets are a problem because antibiotics in your bloodstream have difficulty reaching bacteria trapped inside them. The fluid becomes thicker, more acidic, and harder to drain. This is typically when a drainage procedure becomes necessary.
Stage 3: Chronic Organizing
If the infection persists, the body begins laying down a thick, fibrous peel over the lung surface. This scar tissue can encase the lung like a shell, preventing it from fully expanding. At this stage, the lung is effectively trapped, and surgery is usually the only option to restore normal breathing.
Symptoms to Recognize
Empyema often develops in someone who already has pneumonia, so its symptoms can overlap with or build on top of an existing illness. The hallmark pattern is a pneumonia that initially seems to improve with antibiotics but then worsens, or one that simply fails to get better after several days of treatment.
Common symptoms include persistent fever, chest pain that worsens with breathing, shortness of breath, a productive cough, and general fatigue. Because the infected fluid takes up space in the chest, you may notice that breathing feels progressively more difficult as the fluid accumulates.
How Empyema Is Diagnosed
Imaging is typically the first step. A chest X-ray can reveal fluid in the pleural space, but a CT scan provides much more detail. One of the most reliable signs on a CT scan is the “split pleura sign,” where the two layers of the pleural lining appear separated and thickened around a fluid collection. This sign appears in about 68% of empyema cases and helps distinguish empyema from a lung abscess or a simple non-infected fluid collection.
The definitive diagnosis comes from sampling the pleural fluid with a needle (thoracentesis). Doctors analyze the fluid for several markers. A pH below 7.2, glucose below 60 mg/dL, and LDH levels above 1,000 U/L all point toward pleural infection. If the fluid is visibly thick and cloudy, or if bacteria are found on culture or staining, the diagnosis is confirmed.
Risk Factors
Anyone with pneumonia can develop empyema, but certain conditions raise the risk significantly. Diabetes is one of the most important: in one large surgical cohort, nearly 48% of patients whose empyema recurred had diabetes, compared to about 32% in those who did not have a recurrence. People with diabetes had roughly double the odds of recurrence. Chronic lung disease, alcohol use disorder, weakened immune function, and poor dental health (which harbors bacteria that can reach the lungs) also increase susceptibility.
Delayed treatment is itself a risk factor. When antibiotics are started too late or the wrong antibiotic is chosen, a simple infected pleural effusion has time to progress into full empyema with loculations and fibrous scarring.
Treatment Options
Treatment depends on the stage. In the earliest exudative phase, antibiotics alone may be sufficient. Once pus or loculations have formed, antibiotics need to be paired with drainage to physically remove the infected fluid. Antibiotics cannot adequately penetrate walled-off pockets of pus, so relying on medication alone at this point typically fails.
Chest Tube Drainage
The most common initial approach is placing a chest tube to continuously drain the infected fluid. For straightforward collections, this works well. But when the fluid has divided into multiple pockets separated by fibrin barriers, a standard chest tube may not be enough.
Enzyme Therapy Through the Chest Tube
For loculated empyema, doctors can instill clot-dissolving and DNA-breaking enzymes directly into the pleural space through the chest tube. A landmark trial published in the New England Journal of Medicine found that using these two enzymes together reduced the amount of infected fluid significantly more than placebo. Only 4% of patients receiving the combination needed surgery within three months, compared to 16% in the placebo group. Hospital stays were nearly a week shorter. Importantly, either enzyme used alone was ineffective; the benefit only appeared when both were given together.
Surgery
When drainage and enzyme therapy fail, or when the empyema has reached the chronic organizing stage, surgery becomes necessary. The less invasive option is video-assisted thoracoscopic surgery (VATS), where a camera and instruments are inserted through small incisions to break up loculations and clean out the pleural space. Early referral to surgery gives VATS a better chance of success. If the pleural space is heavily scarred and essentially sealed shut, an open procedure called decortication is needed. This involves peeling the thick fibrous rind off the lung surface so it can expand again. While more invasive, decortication still produces acceptable results in terms of lung function recovery.
Recovery and Outlook
The prognosis for empyema depends heavily on how early it is caught and treated. When diagnosed in the first stage and treated promptly with antibiotics and drainage, most people recover fully. As the disease progresses into later stages, treatment becomes more complex, recovery takes longer, and the risk of complications rises.
The most significant long-term complication is fibrothorax, where scar tissue permanently restricts lung expansion. This can leave someone with reduced lung capacity and chronic shortness of breath, even after the infection itself has cleared. Recurrence is another concern: in a large study of patients treated surgically, about 4.6% experienced a recurrence, typically within about five to six weeks. Diabetes, very low pleural glucose levels, and Streptococcus infections were the strongest predictors of recurrence, with each roughly doubling or tripling the odds.
Because outcomes are so closely tied to timing, the most important takeaway is that a pneumonia that isn’t improving as expected warrants close follow-up. Persistent or worsening symptoms after starting antibiotics, especially new or increasing chest pain and shortness of breath, can signal that fluid is building up in the pleural space and needs direct attention.