Pleural fluid is a thin layer of liquid that sits between the two membranes lining your lungs and chest wall. A healthy adult has roughly 10 mL of it at any given time, just enough to coat the surfaces. Its job is straightforward but essential: it acts as a lubricant so your lungs can expand and contract smoothly with each breath, and it creates a seal of surface tension that keeps your lungs mechanically coupled to your chest wall.
How Your Body Produces and Clears It
Your body continuously makes and reabsorbs pleural fluid to maintain that small, stable volume. Fluid filters out of blood vessels in the pleural membranes at a rate of about 0.6 mL per hour and is absorbed at roughly the same rate by the lymphatic system. This balance is what keeps the volume at around 0.1 to 0.2 mL per kilogram of body weight.
Only the outer membrane (the one lining the chest wall, called the parietal pleura) has lymphatic openings that drain directly from the pleural space. The inner membrane covering the lungs lacks these openings. When the lymphatic system can’t keep up with fluid production, or when fluid is being produced faster than normal, the excess accumulates. That buildup is called a pleural effusion.
What a Pleural Effusion Looks Like
Normal pleural fluid is clear and pale yellow. When something goes wrong, its appearance changes in ways that help doctors identify the problem. Milky or whitish fluid typically signals a chylothorax, where lymphatic fluid leaks into the pleural space. Pus means an infection has set in, a condition called empyema. Bloody fluid is common when the underlying cause is cancer.
Transudates vs. Exudates
When excess pleural fluid develops, doctors classify it into one of two categories based on its chemistry, because the category points toward very different causes.
Transudative effusions happen when the pressures pushing fluid across membranes are thrown off. Heart failure is the most common example. The fluid itself is relatively low in protein and cellular debris because the membranes are intact; they’re just under abnormal pressure.
Exudative effusions result from damage or inflammation to the pleural membranes themselves. Infections, cancers, autoimmune diseases, and tuberculosis all fall into this group. The fluid contains higher concentrations of protein and cellular enzymes because the inflamed or damaged membranes leak more of these substances through.
Doctors distinguish the two types using a set of lab ratios known as Light’s criteria. An effusion is classified as exudative if it meets any one of these thresholds: the protein ratio between the fluid and the blood is 0.5 or higher, the enzyme ratio (using a marker called LDH) is 0.6 or higher, or the fluid’s LDH alone exceeds two-thirds of the upper normal limit for blood. If none of those criteria are met, the effusion is transudative.
What Fluid Analysis Reveals
Beyond the basic transudate-or-exudate distinction, lab tests on pleural fluid can narrow the diagnosis further.
The fluid’s acidity level is one useful clue. When pleural fluid pH drops below 7.30, it consistently points to an exudative process. Six conditions are closely associated with that finding: empyema, cancer, autoimmune disease, tuberculosis, esophageal rupture, and bleeding into the pleural space. A pH below 7.00 narrows the possibilities even further, pointing most often to empyema, autoimmune disease, or esophageal rupture. In cancerous effusions specifically, the pH tends to fall into a narrow range between 7.04 and 7.29.
Cell counts in the fluid tell their own story. A predominance of certain white blood cells suggests acute infection or inflammation, while a predominance of other types leans toward chronic processes like tuberculosis or cancer.
When cancer is suspected, the fluid is examined under a microscope for malignant cells. This cytology test catches about 58% of cancer-related effusions overall, but accuracy varies dramatically by cancer type. Lung adenocarcinoma and ovarian cancer are the easiest to detect this way, with sensitivities around 84% and 85% respectively. Squamous cell lung cancer and mesothelioma are far harder to spot, detected in only about 24% and 29% of cases. Breast cancer falls in the middle at roughly 65%.
How Pleural Fluid Is Collected
The procedure for sampling pleural fluid is called thoracentesis. You sit upright and lean slightly forward with your arms resting on a support, which spreads the ribs apart and gives the doctor easier access. In patients who can’t sit up, the procedure can be done lying down with ultrasound guiding the needle.
After numbing the skin and deeper tissue with a local anesthetic, the doctor inserts a needle over the upper edge of a rib (blood vessels and nerves run along the bottom edge, so going high avoids them). Ultrasound is typically used to locate the fluid and reduce the risk of complications. Once fluid flows back through the needle, a thin flexible catheter slides into place and the needle is removed, allowing the fluid to drain into a collection system.
Most people tolerate the procedure well, but complications can include a collapsed lung (pneumothorax), bleeding, infection, or a drop in blood pressure if a large volume of fluid is drained quickly. The collapsed-lung risk is lower when ultrasound guidance is used throughout.
Common Causes of Fluid Buildup
The list of conditions that produce pleural effusions is long, but a few account for the majority of cases. Heart failure is the leading cause of transudative effusions, because elevated pressure in the heart’s blood vessels forces more fluid across the pleural membranes than the lymphatic system can clear.
Pneumonia and other lung infections are among the most common causes of exudative effusions. Cancer, whether originating in the lung or spreading from elsewhere, is another major cause. Tuberculosis remains a significant source of effusions worldwide. Liver cirrhosis, kidney disease, and pulmonary embolism round out the list of frequent culprits, each disrupting the fluid balance through a different mechanism.