New species-related MIC breakpoints for early detection of development of resistance among Gram-negative bacteria in Swedish intensive care units

The frequency of decreased antibiotic susceptibility among 534 Gram-negativ e aerobic bacilli from patients admitted to intensive care units at eight h ospitals in Sweden during 1997 was evaluated. MICs of cefepime, ceftazidime , ceftriaxone, ciprofloxacin, gentamicin, imipenem and piperacillin-tazobac tam were determined using Etest. Reduced susceptibility (resistant and inte rmediate/indeterminate susceptible strains) was defined according to the MI C breakpoints of the British Society for Antimicrobial Chemotherapy (BSAC), the National Committee for Clinical Laboratory Standards (NCCLS) and the n ew species-related breakpoints of the Swedish Reference Group for Antibioti cs (SRGA). The BSAC/NCCLS/SRGA breakpoints for susceptible category (mg/L) of Enterobacteriaceae are: cefepime, not available (NA)/8/0.5; ceftazidime, 2/8/2; ceftriaxone, NA/8/0.5; ciprofloxacin, 1/1/0.12; gentamicin, 1/4/2; imipenem, 4/4/1; and piperacillin-tazobactam, NA/16/16. The most frequently isolated organisms were Escherichia coil (n = 160; 30%), Klebsiella spp. ( n = 84; 16%), Enterobacter spp. (n = 77; 14%), Pseudomonas aeruginosa (n = 64; 12%) and Proteus spp. (n = 28; 5%). Decreased susceptibility among E. c oli using the BSAC/NCCLS/SRGA respective breakpoints (%) were: cefepime, NA /0/2; ceftazidime, 2/2/2; ceftriaxone, NA/1/2; ciprofloxacin, 2/2/8; gentam icin, 21/0/3; imipenem, 0/0/2; and piperacillin-tazobactam, NA/4/4. Corresp onding levels of decreased susceptibility (%) among Klebsiella spp. were: c efepime, NA/0/5; ceftazidime, 21/1/2; ceftriaxone, NA/1/10; ciprofloxacin, 4/4/19; gentamicin, 25/2/5; imipenem, 0/0/0; and piperacillintazobactam, NA /10/10; and among Enterobacter spp. were: cefepime, NA/1/19; ceftazidime, 3 0/29/30; ceftriaxone, NA/30/36; ciprofloxacin, 3/3/15; gentamicin,18/0/0; i mipenem, 0/0/5; and piperacilllin-tazobactam, NA/27/27. In conclusion, the species-related SRGA breakpoints detected Gram-negative isolates with decre ased susceptibility in comparison with the native population with higher fr equency than did the NCCLS breakpoints. The BSAC breakpoints for susceptibl e organisms were similar to NCCLS for ciprofloxacin and imipenem, and simil ar to SRGA for ceftazidime but lower than both NCCLS and SRGA for gentamici n, causing a much higher frequency of decreased susceptibility to gentamici n.