MIC (Minimum Inhibitory Concentration) Test: What It Is, Why It's Done, and What Your Results Mean

What is the MIC test and what does it tell my doctor about treating my infection?

The Bedaquiline Susceptibility Test by Minimum Inhibitory Concentration (MIC) is a laboratory test that determines how well the antibiotic bedaquiline can stop the growth of tuberculosis (TB) bacteria. This test is specifically used for patients with drug-resistant forms of TB, where standard antibiotics may not work effectively.

How does the MIC test work in the laboratory to measure how much antibiotic is needed to stop bacteria from growing?

The test exposes TB bacteria collected from your sample to different concentrations of bedaquiline in a laboratory setting. Scientists then observe the lowest concentration of the drug that prevents the bacteria from growing. This concentration is called the Minimum Inhibitory Concentration (MIC). A lower MIC means the bacteria are more easily killed by the drug.

Why is the MIC test performed, and when might my doctor order it for me or my child?

This test is primarily performed when a patient has been diagnosed with tuberculosis that is resistant to standard first-line antibiotics. It helps your healthcare provider determine whether bedaquiline will be an effective part of your treatment plan. Bedaquiline is an important antibiotic used specifically for multidrug-resistant tuberculosis (MDR-TB) and extensively drug-resistant tuberculosis (XDR-TB).

How is the sample collected for MIC testing, and what should I expect during the collection process?

For this test, a sample is typically collected from your respiratory system. The most common sample is sputum (mucus from your lungs), which you can cough up in the morning. In some cases, other respiratory samples may be collected, such as fluid from bronchoscopy or tissue samples. For certain types of testing, blood (serum) may also be used to measure drug levels in your body.

What do the results of my MIC test mean, and how will my doctor use them to choose the right antibiotic?

Your test results will show an MIC value, which is compared to established clinical breakpoints. If the MIC is at or below the susceptible breakpoint, the TB bacteria are considered likely to respond to bedaquiline treatment. If the MIC is above the resistant breakpoint, it indicates the bacteria may be resistant to bedaquiline, and your doctor will need to consider alternative treatments. Your doctor will interpret these results in the context of your overall health and other test results.