Andersen 1980.

Study characteristics
Methods	Triple‐blind RCT Country: Denmark Enrolment period: April 1973 to October 1975 Single centre study
Participants	N: 285 participants Age (years; mean (range)): PG: 54 to 56 years (range 21 to 82); CG: 55 years (range 20 to 82) % Men: PG: 72%; CG: 73% Inclusion criteria: direct hernia Exclusion criteria: allergy to penicillin BMI: not reported. Skin antiseptic: not reported. Operative time: not reported. Method of repair: Mc Vay technique (herniorrhaphy) Drain use: removed at postoperative day 4 ASA class: not reported Anaesthesia method: not reported Hospitalisation method: inpatient Length of hospital stay: not reported Surgeon: not reported
Interventions	Prophylactic group (PG): N: 135 Group 1: (n = 59) Ampicillin sodium powder 1 g in the subfascial layers and silk sutures in the fascia and peritoneum Group 2: (n = 76) As Group 1 but polyglycolic acid instead silk sutures Timing of administration: before closure fascial Control group (CG): N: 150 Group 3: (n = 86) No antibiotic therapy and silk sutures Group 4: (n = 64) No antibiotic therapy and polyglycolic acid sutures
Outcomes	Wound infection definition: collection of pus in the wound that requires revision Follow‐up: 1, 3, 6, 12 months SSSI: N infected/N patients (%) PG: 5/135 (3.7%) (Group 1: 1/59; Group 2: 4/76); CG: 6/150 (4%) (Group 3: 3/86; Group 4: 3/64 DSSI: not reported Adverse effects: "none of the patients had an allergic reaction to ampicillin"
Notes	The study was realised in participants who underwent various techniques other that hernia repair, but it was possible to extract the information relating to the patients underwent to hernia repair. Funding was not reported
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Insufficient information available on how "triple‐blind random allocation design" was performed Quote: “patients … who had a direct hernia or gallstones in a gallbladder (shown not to be infected on macroscopic examination) were assigned at the end of the operation to one of the following four regimens: one gram of ampicillin powder … or no antibiotic therapy”
Allocation concealment (selection bias)	Unclear risk	Quote: "...were assigned at the end of the operation to one of the following four regimens:..."
Blinding of participants and personnel (performance bias) All outcomes	Low risk	No information available, but performance bias is unlikely in this setting.
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	No information available
Incomplete outcome data (attrition bias) All outcomes	Low risk	2 patients (out of total of 400 patients) were lost in follow‐up. Available case analysis was performed. The proportion of missing outcomes was considered not enough to have had a clinically relevant impact on the intervention effect estimate.
Selective reporting (reporting bias)	Low risk	Study protocol was not registered, but report contains all expected outcomes, including specification of SSSI
Other bias	Low risk	The study appears to be free of other sources of bias