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So, what are test strips all about?
Immunoassay test strips, like fentanyl test strips (FTS), are quick and low-cost tools used to check drugs for certain substances. They use antibodies to detect a target drug and show the result as one or two lines within minutes. These strips can test for fentanyl, xylazine, benzodiazepines, and other drugs in the unregulated supply.
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Test strips are screening tools, not final confirmation tests. They are very sensitive, so proper mixing and dilution of samples is important. It’s also important to understand their limits and possible false positives or negatives.
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Below is a list of helpful terms related to immunoassay test strips and how well they work and perform.
Cross-Reactivity
Cross-reactivity happens when a test strip reacts to a substance that looks similar to the drug it is meant to find. The antibody sticks to the wrong substance because they have a similar shape. This can cause the test to show a positive result even when the target drug is not present. When this happens, the test is less specific and has a harder time telling different substances apart.
False Negative
A false negative happens when a test strip shows a negative result even though the substance being tested is actually in the sample. This means the test misses something that is really there.
False Positive
A false positive happens when a test strip shows a positive result even though the substance being tested is not actually in the sample. This means the test suggests something is there when it really isn’t.
GC/MS
GC-MS is a lab tool used to find out exactly what active substances are in a drug sample. It turns the sample into a gas and moves it through a long tube. Different substances travel at different speeds, which helps separate them. Each substance leaves a unique “fingerprint” that helps the lab identify it. This method is more sensitive than field testing tools like FTIR and is often used as an extra way to check what is in a drug.
Immunoassay
Immunoassay test strips use special proteins called antibodies to look for a specific drug or related substances. A small amount of liquid sample is pulled up the strip. As the liquid moves, it reacts with the antibodies and creates a result that shows whether the drug may be present or absent.
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LC-QToF
LC-QToF is an advanced lab tool that uses liquid to separate different substances in a sample. It then uses a special detector to measure the weight of each substance very accurately. This tool can find very small amounts of drugs or chemicals, even in tiny samples. It is more sensitive than GC-MS, which means it can detect smaller amounts more easily.
Limits of Detection (LoD)
Limit of detection means the smallest amount of a substance that a test can reliably find. It is the lowest level where the test can still tell the difference between a real sample and a blank or control sample. For test strips, this means the smallest amount of a substance in water that can still be detected with confidence.
Sensitivity
Sensitivity is the ability of a test to correctly identify true positives. High sensitivity indicates the test is good at catching the substance when it’s present.
Specificity
Specificity is the ability to correctly identify true negatives. High specificity indicates the test is good at ruling out a substance when it’s absent, and won’t give many false positives.
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Test Strip Glossary
Accuracy
Accuracy tells us how often a test gets the right answer overall. It includes both true positives (when the test correctly says a drug is present) and true negatives (when the test correctly says a drug is not present). We calculate accuracy by adding the true positives (TP) and true negatives (TN), then dividing by the total number of tests.
False Positive
A false positive happens when a test strip shows a positive result even though the substance being tested is not actually in the sample. This means the test suggests something is there when it really isn’t.
LC-QToF
​LC-QToF is an advanced lab tool that uses liquid to separate different substances in a sample. It then uses a special detector to measure the weight of each substance very accurately. This tool can find very small amounts of drugs or chemicals, even in tiny samples. It is more sensitive than GC-MS, which means it can detect smaller amounts more easily.
Specificity
Specificity is the ability to correctly identify true negatives. High specificity indicates the test is good at ruling out a substance when it’s absent, and won’t give many false positives.
Cross-Reactivity
​Cross-reactivity happens when a test strip reacts to a substance that looks similar to the drug it is meant to find. The antibody sticks to the wrong substance because they have a similar shape. This can cause the test to show a positive result even when the target drug is not present. When this happens, the test is less specific and has a harder time telling different substances apart.
GC/MS
GC-MS is a lab tool used to find out exactly what active substances are in a drug sample. It turns the sample into a gas and moves it through a long tube. Different substances travel at different speeds, which helps separate them. Each substance leaves a unique “fingerprint” that helps the lab identify it. This method is more sensitive than field testing tools like FTIR and is often used as an extra way to check what is in a drug.
Limits of Detection (LoD)
Limit of detection means the smallest amount of a substance that a test can reliably find. It is the lowest level where the test can still tell the difference between a real sample and a blank or control sample. For test strips, this means the smallest amount of a substance in water that can still be detected with confidence.
False Negative
A false negative happens when a test strip shows a negative result even though the substance being tested is actually in the sample. This means the test misses something that is really there
Immunoassay
Immunoassay test strips use special proteins called antibodies to look for a specific drug or related substances. A small amount of liquid sample is pulled up the strip. As the liquid moves, it reacts with the antibodies and creates a result that shows whether the drug may be present or absent.
Sensitivity
Sensitivity is the ability of a test to correctly identify true positives. High sensitivity indicates the test is good at catching the substance when it’s present.