Timing of prophylactic antibiotics in abdominal surgery: Trial of a pre-operative versus an intraoperative first dose

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Abstract
When prophylactic antibiotics are used in abdominal surgery it is customary to give the first dose before the operation. Whilst intra‐operative antibiotics may be effective in elective surgery, there may be an advantage to starting pre‐operatively when there is already an infective focus such as appendicitis. Antibiotics started pre‐operatively (group P) have been compared with antibiotics started after initial abdominal exploration (group T). Three intravenous doses of 500 mg metronidazole plus 1 g cephazolin were given in a randomized, double‐blind study of 700 emergency and elective high‐risk abdominal operations. Antibiotic plasma concentrations at the end of the operation were significantly lower in group P but lay well within the therapeutic range. Wound infection rates, which included minor and delayed infections, were similar in both groups (group P, 57 of 342, 16·7 per cent; group T, 55 of 358, 15·4 per cent; 95 per cent confidence intervals for the difference being −4·1 to +6·7 per cent. In appendicitis, wound infection rates were 12·1 and 13·9 per cent for groups P and T respectively. However, non‐fatal deep sepsis was more common in group P (nine cases) than in group T (two cases) (χ2 = 4·9, P < 0·05). Postoperative infection was twice as common in obese patients whose body mass index (BMI) was ⩾ 26 (39 of 132, 30 per cent) than in thin patients whose BMI was 2 = 13·8, P < 0·001). This study failed to show any advantage to starting antibiotics pre‐operatively, even in appendicitis.