A research report submitted to the Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, in partial fulfillment of the requirements for the degree of Master of Medicine Johannesburg, 2017 / Background: Infections post cardiac surgery can have potentially devastating consequences. Adequate antimicrobial prophylaxis is therefore crucial to limit the occurrence of such complications. Cefazolin is a commonly prescribed prophylactic agent for major cardiac surgery requiring cardiopulmonary bypass (CPB). The effects of CPB on the pharmacokinetic profile of cefazolin are largely unknown. To date there is no published work determining the optimal bolus dosing of cefazolin required to achieve acceptable concentrations intra-operatively during and post CPB. Aim: The aim of this study was to describe the total serum cefazolin levels during elective valve replacement surgery on CPB at CMJAH. Method: A prospective, contextual, descriptive design was used in this study. Cefazolin plasma concentrations were analysed at specific pre-determined time intervals in adults patients scheduled for elective valve replacement surgery. Convenience sampling was used. Results: Sixteen patients were enrolled in the study with equal number of males and females, ranging from 18 years to 59 years of age and with a mean BMI of 28.2 kg/m² (range of 18.1 to 40.2 kg/m²). The mean trough for the unbound concentration of cefazolin was 5.02 μg/ml (range of 2.79 to 10.35 μg/ml). For 5 out of the 16 patients (31.25%) the targeted therapeutic goal of time above MIC (4 μg/ml) of 100% (T > MIC 100%) was not achieved. Seven corresponding pre and post CPB serum samples (A1-A7) were statistically analysed using the paired t-test. The results indicated no statistically significant differences between samples A1-A5 (p = 0.11, 0.34, 0.46, 0.32 and 0.98 respectively). There was a statistically significant difference between the samples A6 and A7 (p = 0.024 and 0.025), however, the clinical significance of these small differences is questionable. Conclusion: Surgical site infections not only result in significant morbidity and mortality but also lead to an increased financial burden to the country’s economy. This study has shown that potentially 31.25% of the patients undergoing cardiac surgery may have an increased risk of acquiring infections due to sub-optimal levels of prophylactic antibiotic during the surgery. In addition, the findings point towards no sequestration of cefazolin in the CPB circuits. / MT2017