Abstract: We included 29 RCTs, involving a total of 4458 participants. We deemed 23 trials to be at high risk of bias in at least one domain. We are uncertain of the effect of a long course of antibiotic prophylaxis (> 24 hours) compared to a short course (≤ 24 hours) on abdominal surgical site infection (RR 1.00, 95% CI 0.81 to 1.23; I² = 0%; 7 studies, 1261 participants; very low-quality evidence), mortality (Peto OR 1.67, 95% CI 0.73 to 3.82; I² = 8%; 7 studies, 1261 participants; very low-quality evidence), or intra-abdominal infection (RR 1.23, 95% CI 0.84 to 1.80; I² = 0%; 6 studies, 111 participants; very-low quality evidence). Based on very low-quality evidence from fifteen studies, involving 2020 participants, which compared different drug regimens with activity against three classes of gastrointestinal flora (gram positive, gram negative, anaerobic), we are uncertain whether there is a benefit of one regimen over another. TSA showed the majority of comparisons did not cross the alpha adjusted boundary for benefit or harm, or reached the required information size, indicating that further studies are required for these analyses. However, in the three analyses which crossed the boundary for futility, further studies are unlikely to show benefit or harm. Very low-quality evidence means that we are uncertain about the effect of either the duration of antibiotic prophylaxis, or the superiority of one drug regimen over another for penetrating abdominal trauma on abdominal surgical site infection rates, mortality, or intra-abdominal infections. Future RCTs should be adequately powered, test currently used antibiotics, known to be effective against gut flora, use methodology to minimise the risk of bias, and adequately report the level of peritoneal contamination encountered at laparotomy.

Plain-language summary: We searched for trials involving participants of any age or sex, who underwent an emergency operation to treat penetrating abdominal trauma. The evidence is current to 23 July 2019. We included 29 studies that included 4458 participants. There were problems with the design and conduct of all of these studies, which means that we were uncertain about the results. Most of these studies were carried out over 20 years ago, using antibiotics that are not often used today. Surgical techniques and practice have also evolved substantially during this time. Seven out of the 29 studies received funding from pharmaceutical companies, whilst the other studies did not state their funding sources. Because of the very low-quality of the evidence, we are uncertain whether giving longer courses of antibiotics after penetrating injury reduces the rate of infections after an operation. We are also uncertain if one antibiotic treatment is better than any other that was tested in the trials. The quality of evidence for all outcomes was very low, mainly due to problems with the way the studies were run. These problems were not using placebos (medications that look identical to the study drug but do not contain the active ingredient), a lack of blinding of both participants or the investigators or inadequate methods of randomly allocating treatments to the participants. There were also key differences in the methods used between the studies. New, better quality studies are required in order to answer questions about the use of antibiotics to reduce infections following penetrating abdominal injury.