In Denmark the most common postoperative pathogen is S. aureus (1), sensitive to dicloxacillin. These bacteria can cause a postoperative infection despite using prophylactic antibiotics. Whether the tissue concentration reached is above the minimal inhibitory concentration (MIC) for the pathogens is unknown, and if lower than expected could result in a postoperative infection. Thus a trial was conducted, measuring the actual tissue concentration of dicloxacillin in human muscle and adipose tissue and compared these to the plasma concentration.

MIC for dicloxacillin against S. aureus was determined using the broth macrodilution method. Six healthy male volunteers aging 25 to 27 years (body-mass-index; 20–28), were recruited. A CMA63 (Mdialysis, Stockholm, Sweden) catheter was placed in the subcutaneous tissue of the abdomen and in the rectus muscle of the thigh and the volunteers given 2 g dicloxacillin intravenously over 5 minutes. In 10 min intervals for the following 6 hours, samples from blood and Microdialysis fluid (flowrate 5 ml/min) were collected. Recovery was determined in vitro. Plasma was isolated from blood samples. The unbound dicloxacillin was isolated from plasma using filter plates (AcroPrep 30K Omega, Pall Corporation, US) centrifuged for 30 minutes at 1000 × g and 37°C. All samples were analyzed with High Performance Liquid Chromatography.

MIC was determined to be 0.125 µg/ml. Average recovery was 73,7 % Maximum concentrations were reached in muscle tissue after a median of 0.5 hours and adipose tissue after 0.8 hours. The geometric mean ration (GMR) of AUC0-6h for adipose tissue compared to plasma was 0.32 [0.15–0.71]. GMR of AUC0-6h for muscle tissue compared to plasma and adipose tissue compared to muscle showed no statistically significant differences. The tissue concentrations were above MIC for 3.4 hours for adipose tissue and 4.1 hours for muscle tissue.

The administration of prophylactic dicloxacillin should be given at least 30 minutes prior to incision to ensure maximum tissue concentrations at the onset of surgery. A second dose should be given after 3.4 hours in case of long surgery time. Since the dicloxacillin concentration reached in the adipose tissue is lower than in plasma, it should be investigated whether this difference is more prominent in adipose patients or patients with impaired peripheral circulation, since these patients are at a greater risk of postoperative infections.