Measurement of antimicrobial susceptibility among invasive isolates of Streptococcus pneumoniae: comparison of the Etest with the standard agar dilution method.

High rates of invasive disease caused by Streptococcus pneumoniae occur in the Alaska Native population. Because of the wide use of empiric antibiotics to treat infection in rural regions of Alaska and concern over the emergence of pneumococcal strains now resistant to an increasing number of antibiotics we compared a simple strip system (Etest) for the measurement of antibiotic susceptibilities to the standard agar dilution method. Eighty-two pneumococcal isolates were tested by both methods. Overall, the Etest MICs of 91% of the isolates agreed within one log2 of the agar dilution, and 99.3% agreed within 2 log2 dilutions. There were no very major or major interpretative category discrepancies with the Etest for any antibiotic tested. There were 4.1% minor interpretative errors with the Etest, which generally occurred at the breakpoint between susceptible and intermediate resistance. The results indicated that the Etest was comparable to the agar dilution method for the measurement of antibiotic MICs for Streptococcus pneumoniae.