Free fluid on CT means that the fluid is free flowing and not encapsulated. Free fluid can be simple, complex or bloody. The quantity of free fluid can range from trace amounts to large amounts filling the abdomen entirely. Sometimes the fluid is located around a structure that is causing the free fluid. Often the consistency, location and amount of free fluid will determine the diagnostic possibilities.
Trace amounts can often be normal, especially in women. Trace amounts of fluid often accumulate in the pelvis in women related to ovulation. This fluid can be simple to bloody. It is often asymptomatic and found on CTs done for other reasons. Trace amounts found in men is less common but can still be normal in some cases. This assumes that no other causes are identified by clinical history or on the CT.
Moderate to large quantities of fluid on CT is usually abnormal. The distinction between small, moderate and large fluid is determined by the radiologist based on his experience. Many times, there will be associated findings on the CT to explain the abnormal fluid accumulation. The clinical history is also often important.
One of the more common causes of moderate to large fluid is liver disease and cirrhosis. This is caused by lowered liver function and increase of pressure in the blood vessels of the liver. In these cases, there will be a history of cirrhosis and often the liver has a cirrhotic appearance with a bumpy irregular outer appearance.
Some patients with cancer will have fluid accumulation in the abdomen. This happens with advanced cancer when it spreads to the lining of the abdomen or the organs. This is most common with ovarian, gastrointestinal and pancreatic cancers. Often there will be a history of cancer, but in some cases this can be the initial presentation of cancer.
Fluid accumulation can also occur when patients are getting peritoneal dialysis through their abdomen. In some cases, inflammatory conditions in the abdomen will cause free fluid of moderate to large quantity. An example would be pancreatitis. In these cases, the fluid builds up around the pancreas. Fluid can also build up around other inflamed structures such as the appendix.
Free fluid can be bloody after trauma, bleeding structure or a surgical complication. In these cases, the fluid build up will be brighter than the usual simple fluid. The history helps in these cases. If there is trauma than an organ injury is likely, such as the liver and spleen. There may be a history of recent surgery. In other cases, the blood will accumulate around whatever structure or organ is bleeding.
Free fluid on CT can therefore represent normal benign finding when it is small, especially in women. Larger amounts of fluid are usually abnormal. The history and any additional findings on CT will often point to the cause. The location, consistency and amount of fluid is also helpful when trying to identify the cause.