Journal of Antimicrobial Chemotherapy, Vol 39, 309-317, Copyright © 1997 by The British Society for Antimicrobial Chemotherapy
IB Ibrahim-Elmagboul and DM Livermore
UK clinical laboratories overestimate ciprofloxacin resistance amongst
Pseudomonas aeruginosa isolates, relative to the MIC breakpoint of 1 mg/L.
Most tests leading to this overestimation use 1 microg discs and are by
Stokes' method with the breakpoint taken as the zone radius for P.
aeruginosa NCTC 10662 minus 3 mm. Aiming to reduce this error rate, we
examined alternative disc breakpoints. Tests were performed for 100 P.
aeruginosa isolates on three media, with breakpoints selected (i) as the
zone for P. aeruginosa NCTC 10662 minus 7 mm, as recommended for
ciprofloxacin by the BSAC; (ii) with reference to MIC/zone correlation
lines; (iii) from natural divisions in zone distribution histograms; and
(iv) so as to minimize categorization errors. Breakpoints from regression
lines, and those optimized to the susceptibility distribution, reduced the
proportion of susceptible organisms misreported as resistant, but the
improvement was not significant (P > 0.05, chi2 test). The breakpoint of
the zone radius for P. aeruginosa NCTC 10662 minus 7 mm significantly
reduced (P < 0.05) the number of susceptible organisms reported as
resistant, but led to 50-75% of those with low level resistance (MIC 2-4
mg/L) and 4-10% of those with high- level resistance (MIC > 4 mg/L)
being classed as susceptible. Irrespective of the medium and the basis of
choosing breakpoints, 5 microg ciprofloxacin discs gave a lower rate of
susceptible organisms being reported as resistant than did 1 microg discs;
however, the improvement was not significant (P > 0.05, chi2 test) and
the 5 microg discs had the disadvantages of forming very large zones for
susceptible isolates and giving some--albeit small--zones for highly
resistant organisms. In conclusion, the over-reporting of resistance could
be reduced by use of zone breakpoints optimized to the MIC distribution and
by the use of 5 microg discs, but the case for these changes is not
overwhelming; taking the breakpoint as the zone for NCTC 10662 minus 7 mm
led to unacceptable numbers of resistant organisms being reported as
susceptible. More fundamentally, ciprofloxacin zones and MICs are
continuously distributed for P. aeruginosa isolates, so susceptibility
tests cannot divide the species into discrete populations. In these
circumstances, it is optimistic to expect disc and MIC categorizations to
agree perfectly.
JOURNAL ARTICLE
Sensitivity testing of ciprofloxacin for Pseudomonas aeruginosa
Department of Medical Microbiology, St Bartholomew's and the Royal London School of Medicine and Dentistry, UK. d.m.livermore@mds.qmw.ac.uk.
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  C. J. Henwood, D. M. Livermore, D. James, M. Warner, and t. Pseudomonas Study Group Antimicrobial susceptibility of Pseudomonas aeruginosa: results of a UK survey and evaluation of the British Society for Antimicrobial Chemotherapy disc susceptibility test J. Antimicrob. Chemother., June 1, 2001; 47(6): 789 - 799. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. M. Livermore and H. Y. Chen Quality of antimicrobial susceptibility testing in the UK: a Pseudomonas aeruginosa survey revisited J. Antimicrob. Chemother., April 1, 1999; 43(4): 517 - 522. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 B. Segatore, D. Setacci, M. Perilli, N. Franceschini, A. De Santis, F. Marchetti, and G. Amicosante Italian Survey on Comparative Levofloxacin Susceptibility in 334 Clinical Isolates of Pseudomonas aeruginosa Antimicrob. Agents Chemother., February 1, 1999; 43(2): 428 - 431. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. Galloway, J. Wright, O. Murphy, and G. Dickinson Sensitivity testing of Pseudomonas aeruginosa to ciprofloxacin: comparison of the modified Stokes' method with MIC results obtained by the Etest J. Antimicrob. Chemother., February 1, 1999; 43(2): 314 - 315. [Full Text] [PDF]
|
|