Spontaneous bacterial peritonitis (SBP) is the development of peritonitis despite the absence of an obvious source for the infection. It occurs almost exclusively in people with portal hypertension (increased pressure over the portal vein), usually as a result of cirrhosis of the liver. It can also occur in patients with nephrotic syndrome. The diagnosis of SBP requires paracentesis (aspiration of fluid with a needle) from the abdominal cavity.