An RDT, or rapid diagnostic test, is a portable medical test designed to detect a specific infection or condition in minutes, without needing a laboratory. You’ve likely used one already: COVID-19 home tests, pregnancy tests, and rapid strep tests at the doctor’s office are all RDTs. They work by detecting target molecules in a small sample of blood, saliva, urine, or nasal fluid, and they display results as visible lines on a small strip or cassette, typically within 15 to 30 minutes.
How an RDT Works
Every RDT relies on the same core principle. A liquid sample (blood, saliva, or nasal fluid) is placed on one end of a small strip, and it travels across the strip by capillary action, the same force that draws water into a paper towel. As it moves, the sample passes through a zone containing antibodies attached to tiny colored particles, usually colloidal gold, which appear as a reddish-pink color. If the target molecule is present in the sample, it binds to these colored antibodies, and the pair travels together along the strip.
Further along the strip, a “test line” contains a second set of antibodies anchored in place. These capture the colored complexes as they pass by, concentrating them into a visible line. A separate “control line” captures leftover colored particles regardless of whether the target molecule is present. This control line confirms the test worked properly. If you see both a test line and a control line, the result is positive. If only the control line appears, the result is negative. If no control line appears at all, the test is invalid and should be repeated with a new kit.
Some RDTs work in reverse. For detecting very small molecules, the presence of the target actually prevents a line from forming. In these “competitive” tests, a missing test line means a positive result. Pregnancy tests and most infectious disease RDTs use the standard format, though, where two lines means positive.
What RDTs Can Detect
RDTs are most commonly used for infectious diseases. The major ones include:
- COVID-19: detects viral proteins from a nasal swab
- Malaria: detects parasite proteins from a finger-prick blood sample
- HIV: detects antibodies in blood or oral fluid
- Influenza (flu): detects viral proteins from a nasal swab
- Strep throat: detects bacterial proteins from a throat swab
Pregnancy tests also use the same lateral flow technology, detecting a hormone in urine. The underlying design is flexible enough that manufacturers can adapt it for nearly any condition where a detectable molecule exists in a body fluid.
How an RDT Is Performed
The exact steps vary by test type, but the process is straightforward. For a blood-based RDT like a malaria test, a health worker cleans your fingertip with an alcohol swab, lets it air dry, then pricks the side of your fingertip with a small lancet. A drop of blood is collected using a tiny loop or capillary tube and transferred to the sample window on the test cassette. A few drops of buffer solution (a liquid that helps the sample flow properly) are added to a second window. Results are typically read after 15 minutes.
For nasal swab tests like COVID-19 or flu RDTs, you or a health worker swirls a swab inside the nostril, dips it into a small vial of solution, then applies drops of that solution to the test strip. The waiting time is similar. Reading results too early or too late can produce inaccurate readings, so following the timing instructions matters.
How RDTs Compare to Lab Tests
The main tradeoff with RDTs is speed versus sensitivity. Lab-based tests like PCR (polymerase chain reaction) amplify tiny amounts of genetic material, making them capable of detecting infections even when very few pathogens are present. In a comparative study of malaria detection methods, PCR identified roughly 13% more cases than RDTs, catching infections that RDTs missed because parasite levels were too low to trigger a visible line.
PCR requires specialized equipment, trained technicians, and often a dedicated laboratory, which makes results take hours to days and costs significantly more per test. RDTs can be performed by people with minimal training, in any setting, for a fraction of the cost. This makes them invaluable in remote areas, field clinics, and home use, where getting a fast answer is more practical than waiting for a lab result. For most people with a symptomatic infection, RDTs are accurate enough to guide treatment decisions on the spot.
Why RDTs Sometimes Get It Wrong
No diagnostic test is perfect, and RDTs have specific vulnerabilities. False negatives, where the test says negative but the person is actually infected, most often happen when the level of the target molecule in the sample is below the test’s detection threshold. This is common early in an infection before the pathogen has multiplied enough, or in people with very mild cases. Improper storage (exposure to heat or humidity), expired test kits, and user error during sample collection also contribute.
False positives happen for different reasons. With malaria RDTs, for example, the target protein can linger in the blood for weeks after an infection has cleared, causing the test to read positive even though the person no longer has active malaria. Cross-reactivity with other infections or with certain proteins produced during autoimmune conditions can also trigger a false positive line. Co-infections with other tropical diseases like dengue or hepatitis C have been documented to cause false positive malaria results.
If an RDT result doesn’t match your symptoms, a follow-up lab test is the standard next step. A negative rapid test when you feel sick is worth retesting after a day or two, since pathogen levels may have risen enough to be detected by then.
RDTs in Global Health
RDTs have transformed healthcare in regions where laboratory infrastructure is scarce or nonexistent. In many parts of sub-Saharan Africa and Southeast Asia, a malaria RDT performed at a village health post may be the only diagnostic tool available. Before widespread RDT use, treatment decisions were often based on symptoms alone, leading to overuse of medications and missed diagnoses of other conditions.
Point-of-care testing with RDTs produces faster results, which enables faster treatment decisions. That said, faster results only improve outcomes when the healthcare system can act on them, providing the right medication and follow-up care. The technology itself is one piece of a larger puzzle that includes supply chains, health worker training, and treatment availability.
Smartphone-based tools are beginning to support RDT use in the field. Apps like Audere’s HealthPulse guide health workers through proper test procedures, photograph the completed test for quality review, and digitize patient data. In field trials, about 89% of malaria RDTs were photographed within the recommended time window, and over 91% of images passed quality checks on the first submission. These tools don’t replace human interpretation but help standardize how tests are performed and documented across large programs.