Kid with post op fever after ruptured appendicitis; what antibiotic best to start on- cefoxetin+metro or just zosyn (piperacillin and tazobactam)?

Both are reasonable: Both are reasonable. The combination gets several different bacteria...The same as zosyn (piperacillin and tazobactam) by itself. The difference is often the cost.

Varies: Treatment for peritonitis after appendectomy for perforated appendicitis is therapeutic. Choice of antibiotic is based on expected bacteria present, patient allergies, cost, availability, and other factors. Cefoxitin and Metronidazole is somewhat duplicative, and zosyn (piperacillin and tazobactam) is an extended-spectrum penicillin. Either regimen is acceptable, or simply Mefoxin alone.

Zosyn (piperacillin and tazobactam): Zosyn (piperacillin and tazobactam) would be better as it covers a broader range of gram negative bacilli as well as enterococcus that would not be covered by cefoxitin/metronidazole. An intraoperative culture would be the best guide to antibiotic treatment. A postoperative abscess is a potential complication that needs to be ruled out if the fever persists.