Zone Size Interpretative Chart For The Kirby-Bauer Test
The Kirby-Bauer Test, also known as the disk diffusion test, is a standard laboratory method used to determine the susceptibility of bacteria to antibiotics. The test is named after two scientists, Kirby and Bauer, who developed the method in 1966. Since then, it has become one of the most widely used methods for antibiotic susceptibility testing.
The Kirby-Bauer Test involves placing small disks containing a specific antibiotic on an agar plate that has been inoculated with a bacterial culture. If the bacteria are susceptible to the antibiotic, a clear zone of inhibition will form around the disk. The size of the zone is then measured and compared to a zone size interpretative chart to determine the susceptibility of the bacteria to the antibiotic.
What is a Zone Size Interpretative Chart?
A Zone Size Interpretative Chart is a reference chart that is used to interpret the results of the Kirby-Bauer Test. The chart provides information on the susceptibility of different bacteria to specific antibiotics based on the size of the zone of inhibition that forms around the antibiotic disk.
The chart is typically provided by the manufacturer of the antibiotic disks and is based on the guidelines set forth by the Clinical and Laboratory Standards Institute (CLSI). The chart provides a range of zone sizes for each antibiotic, with corresponding interpretations of susceptibility, intermediate susceptibility, and resistance.
How to Use a Zone Size Interpretative Chart
Using a Zone Size Interpretative Chart is relatively straightforward. After performing the Kirby-Bauer Test, the size of the zone of inhibition around each antibiotic disk is measured and recorded in millimeters. The results are then compared to the range of zone sizes listed for that particular antibiotic on the interpretative chart.
If the zone size falls within the susceptible range, the bacteria are considered to be susceptible to the antibiotic and the antibiotic is likely to be effective in treating the infection. If the zone size falls within the intermediate range, the bacteria may be susceptible to the antibiotic, but higher doses or longer treatment may be required. If the zone size falls within the resistant range, the bacteria are not susceptible to the antibiotic and an alternative treatment should be considered.
Limitations of the Kirby-Bauer Test
While the Kirby-Bauer Test is a widely used method for antibiotic susceptibility testing, it does have its limitations. One major limitation is that it only measures the ability of bacteria to grow in the presence of a specific antibiotic, and does not take into account other factors that may affect the effectiveness of the antibiotic in treating an infection, such as the location of the infection, the severity of the infection, and the patient's immune system.
Additionally, the test does not differentiate between bacteriostatic and bactericidal antibiotics. Bacteriostatic antibiotics inhibit the growth of bacteria, but do not kill them, while bactericidal antibiotics kill bacteria. The Kirby-Bauer Test does not distinguish between these two types of antibiotics, which can affect the interpretation of the results.
Conclusion
The Zone Size Interpretative Chart is an important tool in interpreting the results of the Kirby-Bauer Test. By providing a range of zone sizes and corresponding interpretations of susceptibility, intermediate susceptibility, and resistance, the chart helps clinicians to determine the most effective antibiotic to treat a bacterial infection. However, it is important to keep in mind the limitations of the test, and to consider other factors that may affect the effectiveness of the antibiotic in treating the infection.