Ascites fluid is an abnormal buildup of fluid inside the peritoneal cavity, the space between the abdominal organs and the abdominal wall. A healthy abdomen contains only a small amount of fluid to lubricate organs as they move, but when disease disrupts the body’s fluid balance, liters of excess fluid can accumulate. Cirrhosis of the liver causes roughly 80% of all ascites cases in the United States.
Why Fluid Builds Up in the Abdomen
The most common pathway starts with liver damage. When the liver becomes scarred (cirrhosis), blood has a harder time flowing through it. This creates a traffic jam in the portal vein, the major vessel that carries blood from the intestines to the liver. That backup, called portal hypertension, forces fluid out of blood vessels and into the abdominal cavity.
At the same time, a damaged liver produces less albumin, a protein that normally keeps fluid inside blood vessels by maintaining osmotic pressure. With less albumin in the bloodstream, fluid leaks more easily into surrounding tissues. The kidneys compound the problem by retaining extra sodium and water in response to signals that the body’s effective blood volume is low, even though fluid is actually accumulating in the wrong place.
Cancer, heart failure, and infections can also drive ascites through slightly different mechanisms. Cancer cells lining the peritoneum block normal fluid drainage and increase the permeability of tiny blood vessels. Heart failure raises pressure in the veins that drain the liver, pushing fluid outward. Tuberculosis and other infections inflame the peritoneal lining itself, causing it to weep fluid.
Common Causes by Frequency
- Cirrhosis: approximately 80% of cases
- Cancer: approximately 10%, most often from ovarian, breast, colon, stomach, pancreatic, or lung cancers
- Heart failure or constrictive pericarditis: about 3%
- Tuberculosis: about 2%
- Kidney disease (nephrotic syndrome or dialysis): about 1%
- Pancreatic disease: about 1%
- Other causes: about 2%, including blood clots in the portal vein, severe malnutrition, and Budd-Chiari syndrome
What Ascites Feels Like
Small amounts of ascites may not cause noticeable symptoms. As fluid increases, you’ll typically notice a gradual swelling of the abdomen, a feeling of fullness or bloating, and tightening of clothes around the waist. Larger volumes can press on the diaphragm, making it harder to breathe, especially when lying flat. Some people experience loss of appetite, nausea, or heartburn because the fluid compresses the stomach. Ankle swelling often accompanies abdominal fluid retention because the same mechanisms that trap fluid in the abdomen also push fluid into the legs.
Doctors check for ascites during a physical exam by tapping on different areas of the abdomen and listening for changes in sound as you shift positions, a technique called shifting dullness. A visible “fluid wave” across the belly is another classic sign. These physical exam findings aren’t perfectly reliable on their own. Shifting dullness has a positive predictive value of only about 51%, meaning it frequently produces false positives. Ultrasound is far more accurate and can detect as little as 100 mL of fluid.
How Doctors Analyze Ascites Fluid
A procedure called paracentesis removes a sample of the fluid through a needle inserted into the abdomen. The first 25 to 30 mL collected goes to the lab for several key tests.
The most important initial measurement is the serum-ascites albumin gradient, or SAAG. This compares the albumin level in your blood to the albumin level in the fluid, both drawn on the same day. A difference of 1.1 g/dL or higher indicates portal hypertension (usually cirrhosis or heart failure) with about 97% specificity. A gradient below 1.1 g/dL points to other causes like cancer, tuberculosis, or pancreatic disease.
The lab also runs a cell count and differential. This is critical for detecting spontaneous bacterial peritonitis (SBP), a dangerous infection of the ascites fluid. SBP is diagnosed when the fluid contains 250 or more neutrophils per cubic millimeter, even if bacterial cultures come back negative. Cultures are sent in blood culture bottles rather than standard tubes because this significantly improves the chance of identifying the responsible bacteria.
The fluid’s appearance provides immediate clues. Clear, straw-colored fluid suggests uncomplicated ascites. Milky fluid signals high triglycerides, a condition called chylous ascites. Red or bloody fluid raises concern for cancer or tuberculous peritonitis. When cancer is suspected, cytology (examining the fluid under a microscope for cancer cells) is ordered, though it only catches about 40% of malignant cases because tumor cells can be scarce or hard to distinguish from inflamed normal cells.
Distinguishing Cancerous From Non-Cancerous Fluid
Because standard cytology misses more than half of malignant ascites cases, doctors sometimes use additional markers in the fluid. Elevated levels of certain enzymes and proteins, particularly ferritin, can help. In one study from the Medical Journal of the Armed Forces India, ascites fluid ferritin above 95 ng/mL detected malignant ascites with 100% sensitivity, though it also flagged some non-cancerous cases. Combining multiple markers (ferritin, cholesterol, and LDH together) improved specificity to 93%, meaning very few false positives, but at the cost of missing some true cases.
No single test definitively rules cancer in or out from a fluid sample alone. Imaging, biopsies, and clinical context all factor into the diagnosis.
Treatment and Fluid Management
Treatment targets both the underlying cause and the fluid itself. For cirrhosis-related ascites, the foundation is reducing sodium intake and using diuretics at the lowest effective dose. Sodium restriction limits the raw material your kidneys use to hold onto water. Diuretics, typically a combination of a potassium-sparing type and a loop diuretic, help your kidneys excrete that excess sodium and water. Doses are adjusted every two to three days based on weight, urine output, kidney function, and electrolyte levels.
When fluid buildup is large or doesn’t respond well to diuretics, therapeutic paracentesis drains the fluid directly. This can remove several liters in a single session. When more than 5 liters are drained, intravenous albumin is given at a dose of 6 to 8 grams for every liter removed. This replacement prevents a drop in blood pressure and protects kidney function. One standardized protocol uses 25 grams of albumin for 5 to 6 liters removed, 50 grams for 7 to 10 liters, and 75 grams for volumes above 10 liters.
For people who need frequent paracentesis, a procedure called TIPS (transjugular intrahepatic portosystemic shunt) can reduce portal pressure by creating a new channel for blood to flow through the liver. This decreases the driving force behind fluid accumulation. In cancer-related ascites, treatment focuses on the underlying malignancy, and drainage may be managed with an indwelling catheter that allows fluid to be removed at home.
Complications to Be Aware Of
The most serious complication of ascites is spontaneous bacterial peritonitis. SBP develops when bacteria migrate from the gut into the ascites fluid, which has limited immune defenses. Symptoms include fever, abdominal pain, and worsening confusion in people with liver disease, though some patients have no obvious symptoms at all. SBP can be life-threatening if not treated promptly with antibiotics, which is why paracentesis is performed whenever there’s clinical suspicion.
Large-volume ascites can also compress the kidneys’ blood supply, contributing to a form of kidney failure specific to advanced liver disease. Fluid that tracks upward through small openings in the diaphragm can cause a pleural effusion (fluid around the lungs), called hepatic hydrothorax, making breathing even more difficult. Hernias are common because the increased abdominal pressure pushes against weak spots in the abdominal wall, particularly around the belly button.