Introduction: Deep infection remains a major complication of joint arthroplasty. Major randomised controlled trials to determine the most effective antibiotic prophylaxis are difficult to construct and interpret. In a conventional theatre most orthopaedic intraoperative wound contamination arrives by the airborne route.

Aim: To use a unique method to compare antibiotics against these airborne bacteria.

Method: Seven antibiotics were tested; Cephradine, Cefuroxime, Cefotaxime, Flucloxacillin, Amoxycillin, Co-amoxyclav and Imipenem. They were incorporated into blood agar at concentrations equivalent to serum levels. Plates were then inoculated with airborne theatre bacteria using a multiple synchronous collection technique. After incubation, the percentage kills were calculated for each antibiotic.

Results: At concentrations equivalent to serum levels one hour following an intravenous dose all of the antibiotics proved highly effective, with kill rates greater than 95%. Imipenem and Co-amoxiclav significantly outperformed the other antibiotics with kill rates of 99.6% and 99.4% respectively. At trough levels the antibiotics achieved kill rates from 61% to 97.6%.

Discussion: Future randomised controlled trials comparing antibiotics in the setting of an already low infection rate are inappropriate. This technique for comparing antibiotic prophylaxis is quick, inexpensive and repeatable. The superiority of Imipenem is not unexpected, given its broad spectrum against both gram positive and negative, aerobic and anaerobic bacteria. Of more interest is the effectiveness of Co-amoxiclav over the presently favoured Cefuroxime.