A sensitivity test most commonly refers to a lab test that identifies which antibiotics can effectively kill or stop the growth of bacteria causing an infection. Your doctor orders one when they need to match you with the right antibiotic rather than guessing. The term also appears in two other contexts: food sensitivity testing and the statistical measure of how accurate a diagnostic test is. Each meaning is distinct, and which one applies to you depends on why you’re searching.
Antibiotic Sensitivity Testing
When you have a bacterial infection, whether it’s a urinary tract infection, a wound infection, or pneumonia, your doctor may order a culture and sensitivity test (often written as “C&S” on lab orders). The lab first grows the bacteria from your sample in a controlled environment. Once the bacteria have multiplied enough to work with, the lab exposes them to a panel of antibiotics to see which ones actually stop the bacteria from growing.
This matters because bacteria increasingly resist common antibiotics. A sensitivity test takes the guesswork out of treatment by showing exactly which drugs will work against your specific infection.
How the Test Works
The most widely used method is called disk diffusion, standardized by Bauer and Kirby in 1966 and still considered the gold standard. A technician spreads bacteria onto a plate, places small discs soaked in different antibiotics on the surface, and incubates the plate overnight. If an antibiotic works, it creates a visible clear zone around the disc where bacteria couldn’t grow. The larger the zone, the more effective the drug.
Another approach measures the minimum inhibitory concentration, or MIC: the lowest amount of an antibiotic needed to prevent bacterial growth. This gives a more precise number than just “works or doesn’t work.” Modern hospitals often use automated systems that can run these tests faster, but the underlying principle is the same.
Reading Your Results
Your sensitivity report will categorize each antibiotic into one of three results:
- Susceptible (S): The bacteria are inhibited at a drug concentration associated with a high likelihood of treatment success.
- Intermediate (I): The drug may work, but the outcome is uncertain. A higher dose or a different route of delivery might be needed.
- Resistant (R): The bacteria aren’t stopped by that antibiotic at normal concentrations, meaning treatment with it would likely fail.
Your doctor uses this report to pick an antibiotic rated “susceptible” for your infection. If you were started on an antibiotic before the results came back, the report may confirm it’s the right choice or prompt a switch to something more effective.
How Long Results Take
Cultures need 24 to 48 hours to grow, and the full sensitivity report can take up to three days. That’s why doctors often prescribe a broad-spectrum antibiotic upfront and then narrow the treatment once results arrive. If your symptoms improve on the initial antibiotic, the sensitivity results serve as confirmation. If they don’t improve, the report becomes essential for course-correcting.
Food Sensitivity Testing
Food sensitivity is biologically different from a food allergy, and this distinction shapes which tests are valid and which are not.
A true food allergy triggers your immune system to produce IgE antibodies against specific food proteins. This reaction is rapid, sometimes occurring within minutes, and can range from hives and swelling to anaphylaxis. Allergists diagnose these with skin prick tests or blood tests that measure IgE levels against specific foods. During a skin prick test, small drops of allergen extracts are placed on your skin and a tiny lancet barely penetrates the surface. If you’re allergic, a raised bump appears at that spot within about 15 minutes.
Food sensitivity (also called food intolerance) is a different process. It doesn’t involve the immune system in the same way. Instead, it typically reflects difficulty digesting certain foods or components, leading to symptoms like bloating, gas, headaches, or fatigue. These reactions tend to be slower, less severe, and harder to pin down.
The Problem With IgG Food Sensitivity Tests
Commercially marketed “food sensitivity tests” usually measure IgG antibodies to various foods. These are widely available online and in pharmacies, but major allergy organizations worldwide have warned against them. The Canadian Society of Allergy and Clinical Immunology, the American Academy of Allergy, Asthma and Immunology, and the European Academy of Allergy and Clinical Immunology all agree: no body of research supports using IgG testing to diagnose adverse reactions to food or predict future reactions.
The reason is straightforward. IgG antibodies to food are a normal marker of exposure and tolerance. Healthy adults and children routinely test positive for IgG against foods they eat without any problems. In fact, studies on oral immunotherapy (treatments that help people overcome allergies) show that rising IgG levels correspond with increased tolerance to a food, not increased sensitivity. A positive IgG result for, say, eggs simply means you’ve eaten eggs. It doesn’t mean eggs are causing your symptoms.
If you suspect food sensitivities, the most reliable approach remains an elimination diet supervised by a dietitian or doctor: you remove suspected foods for a few weeks, then reintroduce them one at a time while tracking symptoms.
Sensitivity as a Test Accuracy Measure
In medical statistics, “sensitivity” describes how well a diagnostic test catches people who actually have a condition. It’s calculated as the number of true positive results divided by everyone who truly has the disease. A test with 95% sensitivity correctly identifies 95 out of 100 people with the condition, missing only 5.
High sensitivity is especially important for screening tests where missing a case could be dangerous. An HIV screening test, for example, is designed to have very high sensitivity so that virtually no infected person gets a false “all clear.” The tradeoff is that highly sensitive tests sometimes flag people who don’t actually have the condition (false positives), which is why a positive screening result often triggers a second, more specific confirmatory test.
If you’ve seen the term “sensitivity” on a lab report or test specification sheet, this is what it refers to: the test’s ability to detect a condition when it’s truly present. A companion metric, specificity, measures the opposite, how well the test correctly identifies people who don’t have the condition. Together, these two numbers tell you how much you can trust a positive or negative result.