Suggested modifications for disk diffusion susceptibility testing criteria for levofloxacin and sparfloxacin following tests with a predictor panel of ciprofloxacin-resistant clinical isolates

Abstract

A predictor panel of 300 clinical bacterial isolates (200 resistant to ciprofloxacin) was used to compare 5-g disk diffusion test results with the MICs of ofloxacin (control), levofloxacin, and sparfloxacin. Regression analysis demonstrated high correlations between the methods for all three fluoroquinolones (r Ն 0.95). In order to minimize disk diffusion testing errors among the fluoroquinolone-resistant strains, the following modifications to previously proposed or published interpretive criteria were suggested: for levofloxacin, susceptible at Ն17 mm (Յ2 g/ml) and resistant at Յ13 mm (Ն8 g/ml); for sparfloxacin, susceptible at Ն20 mm (Յ1 g/ml) and resistant at Յ16 mm (Ն4 g/ml). The study control drug, ofloxacin, did not appear to possess a significant error rate (5% minor error) when fluoroquinolone-resistant strains were tested, and no modifications were proposed. Under these proposed interpretive criteria, the absolute categorical agreements between standardized susceptibility testing methods for levofloxacin and sparfloxacin results were 91.3 and 94.0%, respectively (Յ0.3% major errors and nil very major errors).

Prevalence of resistance to three fluoroquinolones: assessment of levofloxacin disk test error rates and surrogate predictors of levofloxacin susceptibility. AST Surveillance Group

“…The minor error rate (due mostly to oxacillin-resistant staphylococci and nonenteric gram-negative bacilli) was 2.7%, and there were no major errors. Sutton and Jones (26) proposed interpretive criteria 1 mm larger on the basis of a study of 300 isolates, two-thirds of which were resistant to ciprofloxacin. In their study the total error rate was 8.7% by both criteria, but there were two (0.7%) very major errors by the criteria of Pfaller et al (24) but none by their criteria.…”

“…Among the consecutive clinical isolates that we studied, only 7.1% were resistant to levofloxacin, and this should reflect the prevalence of resistance that the average clinical laboratory might expect. When the criteria proposed by Sutton and Jones (26) were applied to the results with these consecutive clinical isolates, the error rates were as follows: very major, 0; major, 1 (0.03%); and minor, 83 (2.4%). In both studies the differences between the two proposed interpretive breakpoints are quite minor and are not compelling, but they both do favor the larger zone diameter breakpoints presented in Fig.…”

“…These parameters for levofloxacin have been assessed, and recommendations for MIC breakpoints, interpretive criteria, and MIC QC limits have been proposed (1,2,24). At least one modifi-cation to the disk diffusion interpretive criteria has also been proposed (26). It has also been suggested that susceptibility to levofloxacin can be predicted by testing ciprofloxacin or ofloxacin and by then applying different interpretive breakpoints (4).…”

“…Scattergrams of levofloxacin MICs and disk diffusion zone diameters for 3,455 consecutive clinical isolates from 10 medical centers. Horizontal lines represent the MIC breakpoints for susceptibility (lower lines) and resistance (upper lines); vertical lines represent the zone diameter breakpoints for susceptibility (right lines) and resistance (left lines) proposed by Sutton and Jones(26). The numbers represent the number of strains at each datum point.…”

Fluoroquinolones: A Class Review

“…Levofloxacin has been approved for the treatment of mild, moderate, and severe acute maxillary sinusitis, acute bacterial exacerbation of chronic bronchitis, community-acquired pneumonia, uncomplicated skin and skin structure infections, complicated urinary tract infections, and acute pyelonephritis caused by susceptible organisms. The tentative MIC break points for levofloxacin are::;2~mL for susceptible organisms, >2 and <8 mL for intermediately susceptible organisms, and~8~mL for resistant organisms (3)(4)(5)(6).…”