A thoracentesis is a medical procedure that removes fluid or air from the space between your lungs and chest wall, known as the pleural space. It serves two purposes: diagnosing the cause of fluid buildup and relieving the breathing problems that come with it. The procedure involves inserting a needle through your back into the pleural space while you sit upright, and it’s typically done in a hospital or clinic setting with local anesthesia.
Why Fluid Builds Up Around the Lungs
Your pleural space normally contains only a thin layer of fluid that helps your lungs move smoothly as you breathe. When excess fluid accumulates there, the condition is called a pleural effusion. This extra fluid compresses the lung, making it harder to expand fully, which causes shortness of breath, chest pain, or a persistent cough.
Many different conditions can trigger this fluid buildup. Heart failure is the most common cause, where increased pressure in the blood vessels around the lungs pushes fluid into the pleural space. Other causes include pneumonia, tuberculosis, cancer (either originating in the lining of the lungs or spreading from elsewhere), blood clots in the lungs, liver cirrhosis, kidney disease, and autoimmune conditions like lupus or rheumatoid arthritis. Chest trauma or recent surgery can also lead to fluid or blood collecting in the pleural space.
Diagnostic vs. Therapeutic Thoracentesis
When doctors don’t know why fluid has collected around your lung, a diagnostic thoracentesis removes a small sample for lab analysis. Testing the fluid helps determine whether an infection, cancer, organ failure, or another condition is responsible. The fluid’s appearance, protein content, cell counts, and culture results all provide clues.
A therapeutic thoracentesis has a different goal: draining enough fluid to help you breathe more easily. In these cases, a larger volume is removed, though providers generally limit drainage to about 1 liter per session. Removing too much fluid too quickly can cause a rare but serious complication called re-expansion pulmonary edema, where the lung struggles to reinflate properly. Sometimes a single procedure serves both purposes, with a sample sent to the lab while a larger volume is drained for symptom relief.
What Happens During the Procedure
You’ll typically sit on the edge of a bed or chair, leaning slightly forward and resting your arms on a pillow or table. This position opens up the back of your rib cage and lets fluid settle to the lowest point, making it easier to access. If you can’t sit up, the procedure can be done while you lie on your side with the affected side facing up and your chest elevated at a 30 to 45 degree angle.
Before anything else, the provider uses ultrasound to locate the fluid and choose the best insertion point. The skin on your back is cleaned with antiseptic, and a sterile field is set up around the area. A local anesthetic (similar to what a dentist uses) is injected to numb the skin and the tissue between your ribs. You’ll feel a brief sting from the numbing injection, but the area should be numb by the time the procedure needle goes in.
The needle is inserted just above the upper edge of a rib to avoid the blood vessels and nerves that run along the bottom of each rib. It passes through the chest wall and into the pleural space, where the fluid is drawn out through a syringe or collected into a drainage bag. The whole process generally takes 15 to 30 minutes. You may feel pressure or a pulling sensation as fluid is removed, and some people develop an urge to cough toward the end as the lung begins to re-expand.
The Role of Ultrasound
Ultrasound has become the standard of care for thoracentesis. The British Thoracic Society recommends that image-guided thoracentesis should always be used to reduce the risk of complications. The difference it makes is substantial: in one meta-analysis comparing the two approaches, ultrasound-guided procedures succeeded in about 97% of patients, compared with 88% using the older landmark-based technique where providers relied on physical exam findings alone.
The safety improvement is even more striking. In a study of over 61,000 thoracentesis procedures, ultrasound reduced the risk of accidental lung puncture by 19%. A separate study of patients with cancer-related fluid buildup found that the rate of pneumothorax (air leaking into the pleural space) dropped from 9% without ultrasound to just 1% with it. Some providers use ultrasound only at the start to mark the best insertion point, while others keep the ultrasound running throughout the entire procedure to track the needle in real time.
Risks and Complications
Pneumothorax is the most common complication. Historically, rates were as high as 19%, but modern techniques have brought this down dramatically. Two recent meta-analyses found an overall rate of about 6%, and experienced operators using ultrasound routinely report rates below 1%. In one review of over 9,300 procedures performed by an experienced provider, the pneumothorax rate was just 0.6%.
Operator experience matters. Less-experienced providers have a pneumothorax rate of about 8.5%, compared with 3.9% for experienced ones. Body size also plays a role: patients who are underweight (BMI below 18) are roughly three times more likely to experience a pneumothorax. Other possible complications include pain at the insertion site, bleeding, and infection, though these are uncommon.
If a pneumothorax does occur, it’s often small and resolves on its own. Larger air leaks may require a chest tube to remove the trapped air and allow the lung to re-expand.
Preparing for a Thoracentesis
For most people, no fasting is required. You can eat and drink normally before the procedure. The main exception is if you take a GLP-1 medication (commonly prescribed for diabetes or weight loss), in which case you’ll need to follow a clear liquid diet the day before and stop all food and drink eight hours before your arrival time.
Blood thinners require special attention. Your provider will tell you whether to stop taking them and how far in advance, which depends on the specific medication and why you’re on it. Don’t stop blood thinners on your own without being told to. If you take insulin or other diabetes medications, check with your provider about dose adjustments. Diuretics (water pills) may also need to be skipped the day of the procedure.
Recovery and What to Expect After
Recovery is quick. A small bandage covers the needle site, and you can remove it after 24 hours. Most people are told to avoid strenuous physical activity for 48 hours. Beyond that, you can return to work, school, and daily routines as soon as you feel ready.
A chest X-ray or brief ultrasound check is sometimes done after the procedure to confirm the lung has re-expanded normally and no air has leaked into the pleural space. If a large amount of fluid was drained, you’ll likely notice an immediate improvement in your breathing. For some conditions, particularly cancer or recurrent heart failure, fluid may build up again over time, and repeat procedures or longer-term drainage solutions may be needed.