Cephalothin and cefazolin in vitro antibacterial activity and pharmacokinetics in dogs.

The periods of time that cephalothin and cefazolin serum concentration remained above minimum inhibitory concentration (MIC) for beta hemolytic, coagulase positive staphylococcal, and Escherichia coli clinical isolates were compared. Cephalothin and cefazolin were similarly very effective in vitro against staphylococcal isolates, with an MIC90 of 0.12 microgram/mL and 0.25 microgram/mL, respectively. In contrast, cefazolin was more effective than cephalothin against E coli isolates; the cefazolin MIC90 for E coli was 16 micrograms/mL and for cephalothin 64 micrograms/mL. Cefazolin (20 mg/kg intravenously [i.v.]) serum concentration remained more than MIC90 for E coli isolates significantly longer than serum concentration of cephalothin (40 mg/kg i.v.) (P < .001).