The choice of the antibiotic for prophylaxis is based on several factors. Always ask the patient about a prior history of antibiotic allergy, as beta-lactams are the commonest type of antibiotics used in prophylaxis. A history of severe penicillin allergy (anaphylaxis, angioedema) means that cephalosporins are also contraindicated, as there is a small but significant risk of cross-reaction.
Most importantly, the antibiotic should be active against the bacteria most likely to cause an infection (Table 2). Most postoperative infections are due to the patient's own bacterial flora. Prophylaxis does not need to cover all bacterial species found in the patient's flora, as some species are either not particularly pathogenic or are low in numbers or both.
It is important to select an antibiotic with the narrowest antibacterial spectrum required, to reduce the emergence of multi-resistant pathogens and also because broad spectrum antibiotics may be required later if the patient develops serious sepsis. The use of 'third generation' cephalosporins such as ceftriaxone and cefotaxime should therefore be avoided in surgical prophylaxis. Often several antibiotics are equal in terms of antibacterial spectrum, efficacy, toxicity, and ease of administration. If so, the least expensive drug should be chosen, as antibiotics for surgical prophylaxis comprise a large portion of hospital pharmacy budgets.
Commonly used surgical prophylactic antibiotics include:
- intravenous 'first generation' cephalosporins - cephazolin or cephalothin
- intravenous gentamicin
- intravenous or rectal metronidazole (if anaerobic infection is likely)
- oral tinidazole (if anaerobic infection is likely)
- intravenous flucloxacillin (if methicillin-susceptible staphylococcal infection is likely)
- intravenous vancomycin (if methicillin-resistant staphylococcal infection is likely).7
Parenteral 'second generation' cephalosporins such as cefotetan have improved anaerobic and aerobic Gram-negative cover compared to first generation cephalosporins. They are sometimes used as a more convenient, but more expensive, alternative to the combination of metronidazole plus either first generation cephalosporin or gentamicin for abdominal surgical prophylaxis.
The bacterial flora in some hospitalised patients may include multi-resistant bacteria such as methicillin-resistant staphylococci. An assessment then needs to be made for each surgical procedure about whether or not prophylaxis with parenteral vancomycin is indicated. Unnecessary use of vancomycin selects for vancomycin-resistant enterococci (VRE), vancomycin-intermediate Staphylococcus aureus (VISA), and vancomycin-resistant Staphylococcus aureus (VRSA), the first two of which already occur in Australian hospitals.
