Types of interpretive errors in susceptibility testing

Abstract

A total of 548 strains of the eleven most common urinary tract pathogens were investigated for possible errors in norfloxacin susceptibility tests comparing MIC determinations with disk diffusion assays. Most strains were found to be sensitive with MIC‐90 values below 1.0 for the Enterobacteriaceae while the classical nalidixic acid resistant species, the gram‐positive bacteria and Pseudomonas aeruginosa, were less susceptible to norfloxacin with MIC‐90 above 1.0 mg/l. MIC‐values close to interpretive MIC‐limits were recorded for S. faecalis and S. agalactiae using the recommendations of the national Committee for Clinical Laboratory Standards (NCCLS) (susceptible, S ≤ 4.0) and for P. aeruginosa and S. aureus using the Swedish Reference Group for Antibiotics (SRGA) standards (S ≤ 1.0). Susceptibility interpretations for these species showed a lack of accuracy consistent with methodological problems of reproducibility, an error called type I. The changes in the MIC‐limits required for these strains to correct the error would be S ≤ 4 for P. aeruginosa and S. aureus, S ≤ 8 for S. agalactiae and S ≤ 0.5 for S. faecalis. A type II error, occurring when a bacterial species shows a regression line different from the regular line, was also identified for S. saprophyticus. The use of breakpoints derived from single strains regression analysis corrected this error and also reduced the frequency of similar misinterpretations in other species. The term “species‐specific MIC‐limits” should be introduced along with the established concept of “species‐specific interpretive zone breakpoints” to allow for the correction of type I interpretive errors. Type II errors can be corrected by using species‐specific interpretive breakpoints, either issued by reference laboratories or derived by calculations from single‐strain regression analysis in the individual laboratory.

Fluconazole and Voriconazole Multidisk Testing of Candida Species for Disk Test Calibration and MIC Estimation

“…In the SRA equation the disk content is retained as a variable, which makes it possible to calculate a regression line for a bacterial species using one single, representative strain and several different disk contents of the antibiotic (15,19). SRA permits the calibration of disk diffusion tests and the calculation of species-related interpretive zone breakpoints (4,19,21,28), as well as the evaluation of an optimal disk potency for susceptibility testing (16,18,21). A modification of the SRA equation, the so called M-test, has shown a potential for MIC estimations (17).…”

Setting interpretive breakpoints for antimicrobial susceptibility testing using disk diffusion

“…In Sweden and some other countries, the standard gentamicin disk content is 30 g, whereas the NCCLS/Clinical and Laboratory Standards Institute (CLSI) recommends a 10 g gentamicin disk for species other than enterococci. Interpretive errors in disk diffusion susceptibility testing have been defined as error types I, II and III, and their remedies discussed, summing up some of the problems mentioned here [50].…”

Calibration of the disk diffusion test for trovafloxacin susceptibility testing of four anaerobic species

“…Bacteroides fragilis and B. thetaiotaomicron showed 21.4 and 31.8% of the strains with an MIC value of 1 mg/L, and 4.5 and 13.6% at 2 mg/L, respectively. This will obviously give rise to interpretive errors of type I [24]. The SRGA interpretive MIC limits for trovafloxacin are more prone to such errors than the NCCLS MIC limits.…”

“…The expected interpretive errors among Bacteroides isolates were therefore confirmed in these disk diffusion tests. For B. thetaiotaomicron the errors are not acceptable and the MIC limits have to be corrected for this species, in a direction determined by relevant clinical trials and correlations to MIC data, tentatively providing zone breakpoints 3 mm lower than the present calculated ones [24]. However, the calculation of interpretive breakpoints using SRA provided an accurate reflection of the present interpretive MIC limits and the method therefore offers a new tool for the individual laboratory to set up a standardized and calibrated disk diffusion method also for anaerobic bacteria.…”