Objective: To compare the in-vitro antimicrobial action of surgical irrigation fluids: 0.9% saline, 2g/L cephradine, 80mg/L gentamicin, 10% povidone-iodine (PVP-I) and 40ppm aqueous iodine (laq) for activity against Staphylococcus aureus, Staphylococcus epidermidis and Escherichia coli in the presence of blood, plasma and saline.

Materials and Methods: 5mL of antimicrobial agent was added to 5ml of blood, plasma or saline containing 10^5–10⁶ CFU/mL of the test bacterium. At 15 seconds, 1 minute, 5 minutes, 1 hour and 2 hours, a 1mL sample was inactivated in 9mL of 0.5% sodium thiosulphate. The bacterial numbers were determined using a biochemical assay (Chemiluminescence) with a calibration curve and by spread plate counts. The data were transformed by a logarithmic function and analysed by linear regression to give 95% confidence intervals for their gradient of change over 2 hours. Significant differences were defined at the 5% level.

Results: In saline. All bacteria were killed within 15 seconds with PVP-I and Iaq, but showed no significant reduction with saline, cephradine or gentamicin. In plasma. E. coli was killed within 15 seconds with all irrigation fluids. S. aureus and S. epidermidis showed no significant reduction with saline, Iaq or cephradine, but did show a significant reduction in the presence of gentamicin. With PVP-I, all S. epidermidis were killed within 15 seconds and all S. aureus within 5 minutes. In blood. E.coli with PVP-I, Iaq and cephradine showed no significant reduction. E. coli with gentamicin did show a significant reduction. Both staphylococci showed a significant reduction over 2 hours with PVP-I and gentamicin, but no significant reduction with saline, cephradine and Iaq.

Conclusions: As blood has a strong chemical inactivating effect on iodine – based formulations, Iaq and PVP-I cannot be recommended for surgical irrigation. Gentamicin should be used in preference to cephradine in surgical irrigation fluids if an antimicrobial agent is required. The residual immunological components (particularly complement) in blood and plasma may enhance the susceptibility of bacteria to antimicrobial agents.