SENSITIVITY TESTING OF CIPROFLOXACIN FOR PSEUDOMONAS-AERUGINOSA

UK clinical laboratories overestimate ciprofloxacin resistance amongst Pseudomonas aeroginosa isolates, relative to the MIC breakpoint of 1 mg/L. Most tests leading to this overestimation use 1 mu g discs and a re 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 10 0 P. aeruginosa isolates on three media, with breakpoints selected (i) as the zone for P. aeruginosa NCTC 10662 minus 7 mm, as recommended f or ciprofloxacin by the BSAC; (ii) with reference to MIC/zone correlat ion lines; (iii) from natural divisions in zone distribution histogram s; and (iv) so as to minimize categorization errors. Breakpoints from regression lines, and those optimized to the susceptibility distributi on, reduced the proportion of susceptible organisms misreported as res istant, but the improvement was not significant (P. 0.05, chi(2) test) . The breakpoint of the zone radius for P. aeruginosa NCTC 10662 minus 7 mm significantly reduced (P < 0.05) the number of susceptible organ isms 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 resistanc e (MIC > 4 mg/L) being classed as susceptible. Irrespective of the med ium and the basis of choosing breakpoints, 5 mu g ciprofloxacin discs gave a lower rate of susceptible organisms being reported as resistant than did 1 mu g discs; however, the improvement was not significant ( P > 0.05, chi(2) test) and the 5 mu g discs had the disadvantages of f orming very large zones for susceptible isolates and giving some-albei t small-zones for highly resistant organisms. In conclusion, the over- reporting of resistance could be reduced by use of zone breakpoints op timized to the MIC distribution and by the use of 5 mu g discs, but th e case for these changes is not overwhelming; taking the breakpoint as the zone for NCTC 10662 minus 7 mm led to unacceptable numbers of res istant organisms being reported as susceptible. More fundamentally, ci profloxacin zones and MICs are continuously distributed for P. aerugin osa isolates, so susceptibility tests cannot divide the species into d iscrete populations. In these circumstances, it is optimistic to expec t disc and MIG categorizations to agree perfectly.