A culture and sensitivity test identifies specific microorganisms causing an infection and determines which medications will be most effective. It is performed when a patient shows signs of infection, such as fever, pain, or swelling, to pinpoint the exact cause for targeted treatment. This test involves two main steps: culturing the sample to grow the microbes and then testing their susceptibility to various antimicrobial agents.
Why the Test is Performed
Doctors order a culture and sensitivity test to identify the microbe (bacteria or fungi) causing an infection. The test also assesses how sensitive these microbes are to different antibiotics or antifungal medications, guiding the most effective therapy.
Knowing which medication will work best helps prevent the use of broad-spectrum antibiotics, which can lead to bacteria developing resistance over time. By targeting the specific microbe with the right drug, this test helps to combat antimicrobial resistance, a growing concern in public health. This targeted approach improves patient recovery and reduces ineffective treatments.
The Test Procedure
The first step is collecting a sample from the patient. The type depends on the suspected infection site: blood for bloodstream infections, urine for UTIs, throat swabs for strep throat, and wound or tissue samples for infected areas.
Once collected, the sample is sent to a laboratory. In the lab, it is placed in growth media (e.g., a petri dish) to encourage microorganisms to multiply. This “culture” phase typically takes 24 to 48 hours for bacteria, though some organisms like fungi or mycobacteria may require several weeks.
After enough microbes have grown, they are subjected to sensitivity testing. This involves exposing the cultured microorganisms to various antibiotics or antifungal medications. Technicians observe which medications inhibit or kill their growth, typically by measuring zones of inhibition around antibiotic-impregnated disks on the culture plate.
Interpreting Test Results
When results are ready, they categorize the microorganism’s response to antimicrobial agents using specific terms: “Susceptible” (S), “Intermediate” (I), and “Resistant” (R). A “Susceptible” result indicates the agent is likely effective at standard dosages, meaning microbial growth is inhibited.
An “Intermediate” result suggests the antimicrobial agent might be effective, potentially at higher doses or if it concentrates well at the infection site (e.g., urine for UTIs).
A “Resistant” result means the microorganism can grow even in the presence of the medication, indicating it will likely not be effective. Doctors use these results to make informed decisions about treatment, choosing medications that show a “Susceptible” rating to ensure the highest chance of success. If no susceptible options are available, a doctor might consider an “Intermediate” drug with careful dosage adjustments or explore alternative treatments. Following prescribed treatment based on these results is important for effective infection resolution and managing antibiotic resistance.