Automated interpretation of disk diffusion antibiotic susceptibility tests with the radial profile analysis algorithm.

An original algorithm referred to as the radial profile analysis algorithm was implemented on a Macintosh Quadra 700 computer to provide an automatic determination of the inhibition zone diameters of antibiotic susceptibility tests performed with the disk diffusion method. After digitization of the petri plate image, each antibiotic disk is recognized and labeled. Pixels of the local zone around each disk are then used for generating a profile pattern that is subjected to decision rules. The resulting estimate of the inhibition zone diameter is then automatically compared with conventional breakpoints for classifying the tested strain in one of the clinical categories of antibiotic susceptibility. The program is also able to request a human reading for some rare plates difficult to interpret. The algorithm accuracy was tested by comparing the results with a combination of independent human measurements performed on the tested plates. The test sample was composed of 98 strains, and 2,552 tests of 40 distinct antibiotics were subjected to the analysis. The difference between the automatic and human diameter estimates was less than 4 mm in 90% of the tests. The agreement between the automatic and human clinical categorizations amounted to 95.5%, and severe (major and very major) disagreements were found in 5.6% of the tests performed with staphylococci but only 0.3% of the tests with gram-negative rods. We conclude that the radial profile analysis algorithm is a solid backbone for an automatic system dedicated to the clinical interpretation of disk diffusion antibiotic susceptibility tests.