Peritonitis: This is a common and serious complication in patients with chronic kidney disease (CKD), especially those on peritoneal dialysis. The presence of fever and abdominal pain in a CKD patient, particularly one undergoing peritoneal dialysis, should prompt a strong consideration for peritonitis due to the direct introduction of dialysate into the peritoneal cavity, which can become infected.

Urinary Tract Infection (UTI): CKD patients are at an increased risk of developing UTIs due to urinary stasis, bladder dysfunction, or the presence of urinary catheters. UTIs can cause fever and abdominal pain, making them a likely consideration in this patient population.
Diverticulitis: Although not exclusive to CKD patients, diverticulitis can cause severe abdominal pain and fever. The risk may be heightened in CKD patients due to potential alterations in bowel habits and increased pressure on the bowel wall.
Gastroenteritis: This can affect anyone but might be more severe or prolonged in CKD patients due to compromised immune function and potential dehydration, which can worsen renal function.

Sepsis: While not a diagnosis in itself, sepsis is a life-threatening condition that can arise from any infection, including those mentioned above. Early recognition and treatment are crucial, making it a "do not miss" diagnosis.
Appendicitis: Although appendicitis is less common in older adults, it can still occur and may present atypically, making diagnosis more challenging. Missing appendicitis can lead to perforation and severe consequences.
Intra-abdominal Abscess: CKD patients may be more susceptible to abscess formation due to impaired immune function. An intra-abdominal abscess can cause fever and abdominal pain and requires prompt drainage.

Hemolytic Uremic Syndrome (HUS): While more commonly associated with diarrhea caused by E. coli O157:H7 in children, HUS can occur in adults and lead to acute kidney injury, which might exacerbate CKD. It's a rare but serious condition that could present with abdominal pain and fever among other symptoms.
Systemic Vasculitis: Certain types of vasculitis, such as ANCA-associated vasculitis, can affect the kidneys and cause CKD. They might also cause fever and abdominal pain, although these are not the most common presenting symptoms.
Abdominal Aortic Aneurysm: Although not typically associated with fever, an abdominal aortic aneurysm can cause abdominal pain and is a surgical emergency if ruptured. CKD patients may have vascular disease, making this a rare but potentially deadly diagnosis to consider.