Patient description: The patient is a 31-year-old woman complaining of abdominal pain. The pain started last night as diffuse abdominal discomfort. She had poor appetite as well as malaise. The pain worsened in intensity and became sharp in the morning. The pain became localized to the right lower quadrant in the morning. The temperature is within the normal limits with normal vital signs. Focal tenderness and guarding were observed during palpation of the right lower quadrant. Palpation of the left lower quadrant causes pain on the right. Her lab work is remarkable for leukocytosis. Computed Tomography of the abdomen with contrast shows the presence of a distended appendix with thickened appendiceal wall without perforation, abscess or gangrene. She is a candidate for laparoscopic appendectomy under general anesthesia.

Clinical trial eligibility criteria: eligible ages (years): 0.25-17.0,  Complicated Intra-abdominal Infections Must be ≥3 calendar months to <18 years of age. Patients aged ≥3 calendar months to <1 year must have been born at term (defined as gestational age ≥37 weeks). 2. Written informed consent from parent(s) or other legally acceptable representative(s), and informed assent from patient (if age appropriate according to local regulations) 3. If female and has reached menarche, or has reached Tanner stage 3 development (even if not having reached menarche) (refer to Appendix E for further details on Tanner staging), the patient is authorised to participate in this clinical study if the following are met: At screening: (i) Patient reports sexual abstinence for the prior 3 months or reports use of at least 1 of the acceptable methods of contraception, including an intrauterine device (with copper banded coil), levonorgestrel intrauterine system (eg, Mirena®), or regular medroxyprogesterone injections (Depo-Provera®); or (b) Patient agrees to initiate sexual abstinence from the time of screening until 7 days after end of treatment with study drug; and (ii) Patient is advised to avoid conception from the time of screening until 7 days after receipt of study drug and agrees not to attempt pregnancy from the time of screening until 7 days after end of treatment with study drug; and (iii) Patient is provided guidelines regarding continuation of abstinence, initiation of abstinence, or about allowed contraception; and (iv) Patient has a negative serum β-human chorionic gonadotropin (β-hCG) test just prior to study entry. Since serum tests may miss an early pregnancy, relevant menstrual history and sexual history, including methods of contraception, should be considered. Note: if the result of the serum β-hCG test cannot be obtained prior to dosing of investigational product, a patient may be enrolled on the basis of a negative urine pregnancy test, though a serum β-hCG test result must still be obtained. 4. Must, based on the judgment of the Investigator, require hospitalisation initially and antibacterial therapy for 7 to 15 days in addition to surgical intervention for the treatment of the current cIAI 5. Require surgical intervention (eg, laparotomy, laparoscopic surgery or percutaneous drainage) to manage the cIAI 6. Must have clinical evidence of cIAI as follows: (i) Pre-operative enrolment 1. Requires surgical intervention that is expected to be completed within 24 hours of enrolment Laparotomy, laparoscopy, or percutaneous drainage 2. Evidence of a systemic inflammatory response (at least 1): Fever (defined as oral temperature >38.5°C, or equivalent to method used) or hypothermia (with a core body or rectal temperature <35°C, or equivalent to method used) Elevated white blood cells (WBC) (>15000 cells/mm3) C-reactive protein (CRP) levels (>10 mg/L) 3. Physical Findings consistent with intra-abdominal infection, such as: Abdominal pain and/or tenderness Localised or diffuse abdominal wall rigidity Abdominal mass 4. Intention to send specimens from the surgical intervention for culture 5. (Optional) Supportive radiologic findings of intra-abdominal infection, such as perforated intraperitoneal abscess detected on: Computed tomography (CT) scan or Magnetic resonance imaging (MRI) or Ultrasound (ii) Intra-operative/postoperative enrolment inclusion(in cases of postoperative enrolment, must be within 24 hours after the time of incision):: Visual confirmation of intra-abdominal infection associated with peritonitis at laparotomy, laparoscopy or percutaneous drainage (to be confirmed pending feasibility); must have 1 of these diagnoses: 1. Appendiceal perforation or peri-appendiceal abscess 2. Cholecystitis with gangrenous rupture or perforation or progression of the infection beyond the gallbladder wall 3. Acute gastric or duodenal perforations, only if operated on >24 hours after singular perforation occurs 4. Traumatic perforation of the intestines, only if operated on >12 hours after perforation occurs 5. Secondary peritonitis (but not spontaneous bacterial peritonitis associated with cirrhosis and chronic ascites)  Involvement in the planning and/or conduct of the study (applies to both AstraZeneca staff and/or staff at the study site) 2. Previous enrolment or randomisation in the present study 3. Participation in another clinical study with an investigational product (IP) during the last 30 days before the first dose of IV study drug or have previously participated in the current study or in another study of CAZ-AVI (in which an active agent was received) 4. History of hypersensitivity reactions to carbapenems, cephalosporins, penicillin, other β lactam antibiotics metronidazole or to nitroimidazole derivatives 5. Concurrent infection, that may interfere with the evaluation of response to the study antibiotics at the time of randomisation 6. Patient needs effective concomitant systemic antibacterials (oral, IV, or intramuscular) in addition to those designated in the 2 study groups (CAZ-AVI plus metronidazole group or meropenem group) (see Section 7.8) 7. Receipt of non-study systemic antibacterial drug therapy for cIAI for a continuous duration of more than 24 hours during the 72 hours preceding the first dose of IV drug, except in proven resistant organisms and/or worsening of the clinical condition for more than 24 hours. More than 2 consecutive doses are not permitted if the individual doses are expected to give >12 hours' cover (ie, giving a total cover of >24 hours.) For patients enrolled after a surgical procedure, only 1 dose of non study antibiotics is permitted postoperatively 8. Patient is considered unlikely to survive the 6 to 8 week study period 9. Patient is unlikely to respond to 7 to 15 days of treatment with antibiotics 10. Patient is receiving haemodialysis or peritoneal dialysis 11. Diagnosis of abdominal wall abscess confined to musculature of the abdominal wall or ischaemic bowel disease without perforation, traumatic bowel perforation requiring surgery within 12 hours of perforation, or perforation of gastroduodenal ulcers requiring surgery within 24 hours of perforation (these are considered situations of peritoneal soiling before the infection has become established) 12. Simple (uncomplicated), non-perforated appendicitis or gangrenous appendicitis without rupture into the peritoneal cavity identified during a surgical procedure OR presence of primary peritonitis (ie, spontaneous bacterial peritonitis) or peritonitis associated with cirrhosis or chronic ascites 13. At the time of randomisation, patient is known to have a cIAI caused by pathogens resistant to the study antimicrobials planned to be used in the study 14. Presence of any of the following clinically significant laboratory abnormalities: 1. Haematocrit <25% or haemoglobin <8 g/dL (<80g/L , <4.9 mmol/L) 2. Serum alanine aminotransferase (ALT) or aspartate aminotransferase (AST) >3×the age-specific upper limit of normal (ULN), or total bilirubin >2×ULN (except known Gilbert's disease) For a) to b): unless if these values are acute and directly related to the infectious process being treated. 15. Creatinine clearance<30 mL/min /1.73 m2 calculated using the child's measured height (length) and serum creatinine within the updated "bedside" Schwartz formula (Schwartz et al, 2009): CrCl (mL/min/1.73m2)=0.413×height (length) (cm)/serum creatinine (mg/dL) 16. History of seizures, excluding well-documented febrile seizure of childhood 17. Any situation or condition that would make the patient, in the opinion of the Investigator, unsuitable for the study (eg, would place a patient at risk or compromise the quality of the data) or may interfere with optimal participation in the study 18. If female, currently pregnant or breast feeding