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

A multicentre, open label, two-arm, parallel group, pragmatic, randomised controlled trial with internal pilot. A total of 1166 consenting adult patients with cIAI will be recruited and randomised on a 1:1 basis between 28-days antibiotics and standard care antibiotics. Patients will be followed up for 180 days to determine cost effectiveness and the rate of treatment failure in each group.

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 (\