Ascites is the medical term for abnormal accumulation of fluid inside the peritoneal cavity, the space between the abdominal wall and the organs within it. In about 80% of cases in the United States, it results from cirrhosis of the liver. The remaining cases are split among cancers (about 10%), heart failure (about 3%), and less common conditions like tuberculosis or kidney disease.
How Fluid Builds Up
Under normal circumstances, a small amount of fluid circulates through the peritoneal cavity to lubricate the organs. Ascites develops when the balance between fluid entering and leaving that space breaks down. In cirrhosis, the most common pathway starts with rising blood pressure in the portal vein, the major vessel that carries blood from the intestines to the liver. Scar tissue in the liver blocks normal flow, and pressure backs up.
That increased pressure triggers blood vessels in the gut to widen, which paradoxically worsens the problem. The body senses a drop in effective blood volume and responds by retaining sodium and water through the kidneys. At the same time, the damaged liver produces less albumin, a protein that normally keeps fluid inside blood vessels. With less albumin pulling fluid back in and more pressure pushing it out, fluid weeps through vessel walls into the abdominal cavity. The lymphatic system tries to compensate by draining the excess, but eventually it gets overwhelmed.
Causes Beyond Liver Disease
While cirrhosis dominates, ascites has a long list of possible causes. Peritoneal cancers account for roughly 10% of cases, most often originating from ovarian, breast, colon, stomach, lung, or pancreatic tumors. In about 20% of cancer-related cases, the primary tumor is never identified. Heart failure, particularly when it raises pressure in the veins returning blood to the heart, causes about 3% of cases. Rarer causes include kidney disease, pancreatitis, and infections like tuberculosis affecting the abdominal lining.
The cause matters because it shapes treatment and outlook. Doctors use a simple lab calculation called the serum-ascites albumin gradient (SAAG) to sort this out. A sample of the abdominal fluid is drawn and compared to a blood sample. If the difference in albumin concentration is 1.1 g/dL or higher, the ascites is almost certainly related to portal hypertension, usually from liver disease. A value below that threshold points toward other causes like cancer or infection.
Grading and Detection
Ascites is classified into three grades based on how much fluid has accumulated. Grade 1 is mild, detectable only by ultrasound imaging. Grade 2 is moderate, visible as symmetrical swelling of the abdomen. Grade 3 is large or “tense” ascites, with marked distension that can make breathing difficult and eating uncomfortable.
Physical examination has real limitations in detecting early ascites. Shifting dullness, a technique where a doctor taps on the abdomen while you change position, requires roughly 1,000 mL (about a quart) of fluid before it becomes noticeable. Standard percussion of the abdomen generally doesn’t pick up fluid until there’s more than 1,500 mL present. That means Grade 1 ascites is reliably caught only on imaging, which is why ultrasound plays such a central role in diagnosis.
What Ascites Feels Like
Small amounts of fluid may cause no symptoms at all. As fluid increases, you’ll typically notice your abdomen getting larger, sometimes rapidly. Pants that fit last week may suddenly feel tight. Moderate to large volumes create a sensation of fullness and pressure, often making it hard to eat a normal-sized meal. Some people feel short of breath, especially when lying flat, because the swollen abdomen pushes upward against the diaphragm. Ankle swelling frequently accompanies abdominal fluid buildup, since the same mechanisms driving fluid into the belly also cause fluid retention in the legs.
How Ascites Is Managed
For cirrhosis-related ascites, the first step is reducing sodium intake. Guidelines recommend limiting daily salt to under 2 grams of sodium, roughly equivalent to less than a teaspoon of table salt. In practice, this means avoiding processed and precooked foods, which are the biggest sources of hidden sodium. Nutritional counseling helps, because most people dramatically underestimate how much sodium they consume.
Diuretics, medications that help the kidneys excrete more sodium and water, are the backbone of medical treatment. For a first episode of moderate ascites, doctors typically start with a single diuretic that blocks a hormone called aldosterone, which drives sodium retention. If the ascites is severe or keeps coming back, a second diuretic is added to increase water output. Nearly half of patients on diuretics experience side effects like electrolyte imbalances or kidney function changes, so regular blood work is part of the routine. Fluid restriction (limiting how much you drink) is generally not necessary unless blood sodium levels drop very low.
When fluid accumulates faster than medications can remove it, a procedure called large-volume paracentesis is performed. A needle is inserted through the abdominal wall under local anesthesia, and fluid is drained directly. “Large volume” typically means removing more than 5 liters at once. This provides rapid relief from pressure, pain, and shortness of breath. After draining that much fluid, an albumin infusion is given through an IV to prevent a drop in blood pressure and protect kidney function.
What Ascites Means for Prognosis
The appearance of ascites in someone with liver disease is a significant turning point. It marks the transition from what doctors call compensated cirrhosis, where the liver is damaged but still managing, to decompensated cirrhosis, where the liver is failing to keep up. A population-based study of older Americans found that median survival after developing ascites was about 1 year. For those who needed repeated fluid drainage procedures, median survival dropped to less than 5 months.
These numbers reflect the seriousness of the underlying disease rather than the fluid itself. Ascites is a symptom, not a standalone diagnosis, and its prognosis depends heavily on what’s causing it and how that cause responds to treatment. In cirrhosis, liver transplant evaluation often begins once ascites develops, because it signals that the liver’s decline has reached a critical stage.