Spontaneous Bacterial Peritonitis (SBP): This is the most likely diagnosis given the context of SBP, as it directly matches the condition being considered. SBP is an infection of the ascitic fluid in the absence of any surgically treatable source of infection or any evidence of bowel perforation. It is a common complication in patients with cirrhosis and ascites.

Secondary Peritonitis: This condition occurs due to the perforation of the gastrointestinal tract or other abdominal organs, leading to infection of the peritoneal cavity. It's a likely consideration because the presentation can sometimes mimic SBP, especially if the perforation is small or contained.
Tuberculous Peritonitis: In areas where tuberculosis is common, tuberculous peritonitis can present similarly to SBP, with ascites and abdominal symptoms. It's an important differential, especially in patients with risk factors for tuberculosis.

Intra-abdominal Surgery or Trauma: Conditions requiring immediate surgical intervention, such as a ruptured appendix or intestinal perforation, can present with abdominal pain and signs of peritonitis. Missing these diagnoses can be fatal.
Ectopic Pregnancy: Although less common in the context of SBP, an ectopic pregnancy can cause abdominal pain and peritoneal signs, especially if there is rupture. This diagnosis is critical to consider in women of childbearing age.
Mesenteric Ischemia: This condition involves insufficient blood flow to the intestines and can present with severe abdominal pain out of proportion to physical findings. It requires prompt diagnosis and treatment to prevent bowel infarction.

Pneumococcal Peritonitis: This is a rare cause of peritonitis, often associated with pneumonia or other infections. It's less common than SBP but should be considered, especially in patients with known pneumococcal infections.
Fungal Peritonitis: Typically seen in immunocompromised patients or those on long-term antibiotic therapy, fungal peritonitis is a rare but serious condition that requires specific antifungal treatment.
Neoplastic Peritonitis: Malignant cells can infect the peritoneal cavity, either from a primary peritoneal cancer or metastasis from another site. This condition is rare and usually presents with more chronic symptoms, but it should be considered in the differential diagnosis of peritonitis, especially in patients with known cancer.