When your lungs can’t expand fully because fluid is squeezing them, breathing becomes a struggle. That’s pleural effusion-fluid building up between the layers of tissue surrounding your lungs. It’s not a disease itself, but a sign something else is wrong. About 1.5 million people in the U.S. get it every year. Some cases are mild and go away on their own. Others are serious, even life-threatening. The key isn’t just removing the fluid-it’s finding out why it’s there and stopping it from coming back.

What Causes Pleural Effusion?

Pleural effusions fall into two main types: transudative and exudative. The difference matters because it tells you what’s going on inside your body.

Transudative effusions happen when fluid leaks through healthy blood vessels because of pressure changes. The most common cause? Congestive heart failure. It accounts for about half of all transudative cases. When your heart can’t pump well, blood backs up, pressure builds, and fluid pushes into the pleural space. Other causes include cirrhosis of the liver and nephrotic syndrome-where your kidneys leak protein into your urine, lowering the pressure that keeps fluid in your blood vessels.

Exudative effusions are more serious. They happen when inflammation, infection, or cancer damages blood vessels, making them leaky. Pneumonia is the top cause-responsible for 40 to 50% of these cases. Cancer comes in second, making up 25 to 30%. Lung cancer, breast cancer, and lymphoma are the usual suspects. Pulmonary embolism (a blood clot in the lung) and tuberculosis are less common but still important to rule out.

Here’s what doctors look for to tell the difference: Light’s criteria. These are three simple tests on the fluid:

• Pleural fluid protein divided by blood protein > 0.5
• Pleural fluid LDH divided by blood LDH > 0.6
• Pleural fluid LDH > two-thirds the upper limit of normal blood LDH

If any one of these is true, it’s exudative. These criteria are 99.5% accurate at catching exudative effusions. Missing one can mean missing cancer or an infection.

Why Thoracentesis Is Necessary

If you have more than 10mm of fluid on an ultrasound, or you’re short of breath, doctors will likely do a thoracentesis. This is the procedure to drain the fluid using a needle or thin tube. It’s not just to make you feel better-it’s to find out what’s causing it.

Ultrasound guidance is now standard. Ten years ago, doctors sometimes guessed where to stick the needle. Now, they use real-time imaging. That cuts the risk of a collapsed lung (pneumothorax) from nearly 19% down to just 4%. That’s an 80% drop in complications.

The needle usually goes between the 5th and 7th ribs on your side, near the armpit. For diagnosis, they take 50 to 100 milliliters-enough for lab tests. For relief, they can remove up to 1,500 milliliters in one session. But they don’t drain everything at once. Removing too much too fast can cause re-expansion pulmonary edema-a rare but dangerous condition where fluid floods the lung after it reopens.

The fluid gets tested for:

• Protein and LDH (to classify transudate vs. exudate)
• Cell count (to spot infection or cancer cells)
• pH (below 7.2 means a complicated infection)
• Glucose (below 60 mg/dL suggests empyema or rheumatoid arthritis)
• Cytology (to look for cancer cells)
• Amylase (high levels point to pancreatitis)
• Hematocrit (if it’s over 1%, it could mean a blood clot or pneumonia)

Cytology finds cancer in about 60% of malignant effusions. But even if it’s negative, cancer can still be there. That’s why doctors don’t stop at one test-they keep looking.

How to Stop It From Coming Back

Draining the fluid once doesn’t fix the problem. If you don’t treat the cause, it comes back. And it often does.

For malignant effusions: If cancer caused it, recurrence within 30 days after just draining is about 50%. That’s why doctors move fast. The two main options are pleurodesis and indwelling pleural catheters.

Pleurodesis means scarring the pleural space shut. Talc is the most effective agent-it works in 70 to 90% of cases. But it’s painful. Up to 80% of patients need strong pain meds after. It also requires a hospital stay.

Indwelling pleural catheters are changing the game. These are small tubes left in place for weeks. You drain the fluid yourself at home, usually once or twice a week. Success rates hit 85 to 90% after six months. Hospital stays drop from 7 days to just 2. And patients report better quality of life. This is now the first choice for many with advanced cancer.

For heart failure: Drain the fluid, sure-but the real fix is treating the heart. Diuretics like furosemide are the main tool. But the best results come when doctors use NT-pro-BNP blood levels to guide treatment. When levels drop, fluid stays away. Recurrence drops from 40% to under 15% in three months.

For pneumonia-related effusions: Antibiotics are essential. But if the fluid turns thick, cloudy, or has a low pH (below 7.2) or low glucose (below 40 mg/dL), it’s becoming an empyema-a pus-filled infection. That’s a surgical emergency. If you don’t drain it, 30 to 40% of cases turn into empyema. Ultrasound helps spot these changes early.

After heart surgery: About 1 in 5 people get fluid after bypass surgery. Most clear up on their own. But if more than 500 milliliters drains per day for three days straight, doctors will leave a chest tube in longer. This prevents recurrence in 95% of cases.

What Doesn’t Work

Not every fluid collection needs to be drained. Studies show that 30% of thoracentesis procedures on small, asymptomatic effusions provide no benefit. They don’t help diagnosis. They don’t improve breathing. They just add risk.

Doctors are also moving away from chemical pleurodesis for non-cancer effusions. There’s no good evidence it helps for things like liver or kidney disease. The risks outweigh the benefits.

And while thoracentesis is common, it’s not a one-size-fits-all fix. The best approach is personalized. A 70-year-old with lung cancer and poor mobility needs a different plan than a 45-year-old with pneumonia and no other health issues.

The Big Picture

The biggest shift in the last decade? Treating the cause, not just the symptom. Dr. Richard Light, who created the criteria we still use today, said it best: “Treating the effusion without treating the cause is like bailing water from a sinking boat without patching the hole.”

Today, we have better tools. Ultrasound. Indwelling catheters. Biomarkers like pH and NT-pro-BNP. We know now that early, targeted action saves lives. For malignant effusions, 5-year survival has doubled since 2010-from 10% to 25%. That’s not because we drain more. It’s because we treat smarter.

Don’t ignore breathlessness. Don’t assume it’s just aging. If you’ve had unexplained shortness of breath for more than a few days, get it checked. A simple ultrasound and fluid test can catch cancer, infection, or heart failure before it’s too late.

What to Do Next

If you’ve been diagnosed with pleural effusion, ask your doctor:

• Is this transudative or exudative? What’s the likely cause?
• Have you tested the fluid for pH, glucose, and cytology?
• Is ultrasound guidance being used?
• What’s the plan to prevent recurrence based on the cause?

Don’t accept a simple drainage without knowing why it happened. That’s like fixing a leaky roof without checking the pipes underneath. With today’s tools, we can find the source-and stop it for good.