EXTENDed Antibiotic Durations Compared to Standard Durations for Patients With Complicated Intra-abdominal Infection.

Inclusion Criteria: * Adults (≥ 16 years) with cIAI\* (see cIAI definition) * Being treated with antibiotics until the point of randomisation, but within 10 days of initiation of effective antibiotic treatment\*\* for cIAI * Ability to provide informed consent by the patient or their consultee. * More than 72 hours\*\*\* of active in-patient management for the patients cIAI is required * In the event that the patient is re-admitted to hospital during the trial period, they are likely to be admitted to a hospital participating in the EXTEND trial. Patients will be included in the trial whether or not they undergo surgical or radiological source control procedures. \* cIAI is defined by the following case definition: * A clinical presentation consistent with cIAI, plus * Fever (temperature of ≥ 37.8°C) and/or a neutrophilia (\> 7.5×109/L) and/or neutropaenia (\<1.8 x 109 /L) and/or intestinal pathogens cultured from sterile sites (closed peritoneum or blood) around the time of cIAI diagnosis, plus * Evidence of pathologic findings on radiologic examination, or * Evidence of pathologic findings at operation \*\* The first day of effective antibiotic treatment will be determined by the patient's clinical team or clinical research team. Antibiotics that do not count towards these 10 days of effective treatment are: * Antibiotic prophylaxis e.g., penicillin for splenectomy, elective surgery antibiotic prophylaxis, UTI prophylaxis * Treatment for other infections that is not …