Plasma, bone, hip capsule and drain fluid concentrations of ampicillin and flucloxacillin during total hip replacement after intravenous bolus injection of magnapen.

1. A rapid intravenous bolus injection of 4.0 g Magnapen (which contains 2.0 g of ampicillin and 2.0 g of flucloxacillin) was to seven patients undergoing total hip replacement immediatly before induction of general anesthesia. Postoperatively the patients patients received 2.0 g Magnapen by intramuscular injection every 6 h for up to 72 h until removal of the wound drains. 2. The plasma, bone, hip capsule and drain fluid concentrations of ampicillin and flucloxacillin were measured by a differential small plate microbiological assay method using Sarcina lutea and a penicillinase producing Staph. aureus Russell as the test organisms. 3. The mean +/- s.e. mean concentrations of ampicillin after this regimen were 4222.2 +/- 285.0 microgram/ml (plasma), 65.6 +/- 1.3 microgram (g (hip capsule), 19.1 +/- 3.8 microgram/g (cancellous bone), and 211.1 +/- 65.6 microgram/g (ground up bone) respectively. 4. The mean +/- s.e. mean flucloxacillin concentrations after this regime were 137.2 +/- 28.4 microgram/ml (plasma), 61.8 +/- 15.0 microgram/g (hip capsule), 47.1 +/- 9.5 microgram/g (cancellous bone) and 139.4 +/- 21.8 microgram/g (ground up bone) respectively. 5. An intravenous bolus injection of Magnepen (4.0 g), given immediately before induction of general anaesthesia, provides concentrations of ampicillin and flucloxacillin in plasma, hip capsule, cancellous and ground up bone, and drain fluid that exceed the MICs of these antibiotics against Staph. aureus and E. coli. 6. The plasma, hip capsule, cancellous and ground up bone concentrations of ampicillin after this dose of Magnapen do not, however, exceed the MICs of the Gram negative anaerobes that sometimes cause postoperative wound infections in these patients.