Before a drug is chosen, the lab tests the organism from this one person against a panel of antibiotics. Three methods do that job. The Kirby–Bauer plate measures a clear zone. The Etest strip and the microdilution tray read out a real MIC. Pick an isolate, pick a drug, and read all three. Then compare each result with its breakpoint and sign it out as S, I, or R.
The platetap a disk
The report that goes to the charttap a row
A zone is in millimeters across. An MIC is in micrograms per milliliter. S means the drug should work. I means it sits on the line. R means it is not a reliable choice.