Case Presentation: An 11-year-old boy with a history of mesenteric adenitis and appendectomy presented for abdominal pain two days after bruising his left flank by running into a countertop. ROS was positive for a new rash on the soles of his feet, recently noticed bubbles in urine, headache, and nausea, but negative for facial rash, oral ulcers, chest pain, joint pain, or hematuria. No family history of autoimmune or renal disease. His parents had COVID-19 two months prior to presentation, but he tested negative around the time of their diagnoses and has remained asymptomatic. Vitals: Temp 38 C, BP 139/79, HR 125 bpm, RR 18, SpO2 100% on room airExam: Uncomfortable, avoids any movement. No oral lesions. Tachycardic, no murmur or rub. No adventitious breath sounds. 2+ pitting edema of hands and ankles. Diffuse rebound tenderness of abdomen. Left flank ecchymosis. Annular erythematous rash with central clearing and scaling perimeter on soles of feet.Initial labs/imaging:Hgb 10.6 g/dL, Plt 132×10^9/L, lymphocyte count 0.8×10^9/LBUN 25 mg/dL, Cr 0.7 mg/dL, albumin 2.2 g/dL, CRP 28.2 mg/L [ref <10 mg/L]C3/C4 undetectableUrine: Large blood (>100 RBCs), protein>300 mg/dl, UP/C 5.8CT abdomen/pelvis: Moderate ascites, small bilateral pleural effusions. No bowel thickening.Due to concern for a vasculitis and peritonitis he was admitted for a renal biopsy and diagnostic paracentesis. He was empirically treated for bacterial peritonitis with cefotaxime while awaiting procedural sedation. Ascitic fluid with 603 neutrophils/mm^3. Methylprednisolone was started while awaiting biopsy results, with symptomatic improvement. Peritoneal culture was ultimately negative, but since obtained after antibiotic initiation, a five day antibiotic course was completed. Renal biopsy confirmed Class IV lupus nephritis. Positive serologies included ANA 1:640, DS-DNA 1:1280, anti-SM, anti-RNP and anti-SCL-70.

Discussion: It is important to have increased suspicion for lupus in a pediatric patient with peritonitis. Vague abdominal pain is common in pediatric SLE, often attributed to lupus enteritis, gastroenteritis, pancreatitis, or constipation(1,2). Ascites can be due to associated nephrotic syndrome, but clinical peritonitis seems to be recognized less frequently than intrathoracic serositis. Peritonitis as a manifestation at the initial presentation is particularly uncommon(3,4). Lupus peritonitis may have a distinctive laboratory profile, with decreased ascitic complements and negative culture(4,5,6). Our patient had a CT scan to evaluate for intra-abdominal injury; however, a perceived acute abdomen could subject a patient to excess or unnecessary radiography and surgery(7).

Conclusions: Lupus peritonitis is a rare presentation of lupus in both pediatric and adult medicine. Particularly in pediatrics, there should be a high suspicion for a systemic inflammatory or autoimmune disorder in a patient with peritonitis and consideration of treatment with steroids and antibiotics until a diagnostic paracentesis can be pursued.