Susceptibility Testing – What It Is and Why You Need It

When you see a patient with an infection, the first question is usually “Which drug will actually work?” Susceptibility testing (often called AST for Antibiotic Susceptibility Testing) gives you that answer. It’s the lab’s way of checking whether a bug is resistant or sensitive to a specific medicine, so you can avoid guessing and prevent treatment failures.

Why Susceptibility Testing Matters

Imagine prescribing a drug that the bacteria are already immune to. Not only does the patient stay sick, but you also add pressure that pushes the bug to become even more resistant. By getting a quick susceptibility report, you can choose an effective drug, lower side‑effects, and cut down on unnecessary hospital stays. In short, it saves time, money, and lives.

Besides helping individual patients, the data from these tests feeds into larger surveillance programs. Hospitals track resistance trends, public health agencies spot emerging threats, and drug companies use the information when developing new antibiotics.

How the Test Is Done

Most labs use one of two simple methods: disk diffusion (the classic Kirby‑Bauer test) or automated broth microdilution. In disk diffusion, a petri dish full of the bacteria gets paper disks soaked in different antibiotics. After incubation, you measure the clear zones around each disk – bigger zones mean the bug is sensitive.

Broth microdilution puts tiny amounts of antibiotics into wells with liquid growth media. The lowest concentration that stops bacterial growth is the MIC (minimum inhibitory concentration). The lab then compares that MIC to standardized breakpoints to label the organism as susceptible, intermediate, or resistant.

If you’re ordering the test, make sure the sample is fresh and the request is clear. Tell the lab the suspected organism and any relevant clinical info. Some labs can also do rapid molecular tests that detect resistance genes in a few hours, which is handy for serious infections like sepsis.

Once the report arrives, look for the “S” (susceptible) column. If the bug is resistant to first‑line drugs, you’ll need to move to a broader‑spectrum antibiotic or consider combination therapy. Always double‑check dosing recommendations – the same drug can act differently depending on the infection site.

Finally, remember to document the susceptibility results in the patient’s chart and inform the pharmacy. This way, the whole care team stays on the same page, and you reduce the risk of duplicate or inappropriate prescriptions.

Susceptibility testing isn’t a mystery; it’s a routine step that turns guesswork into science. Use the results, stay updated on local resistance patterns, and you’ll keep your patients on the right track faster than ever.