Rapid diagnostic tests give medical information quickly at a clinic, pharmacy, school, airport, or home. Many of these tests use a lateral-flow device, which is the same general format used in many COVID-19, flu, strep, malaria, and pregnancy tests. Their main value is speed because results often appear in 10 to 30 minutes without a full laboratory setup.
Fast results can guide treatment, isolation, follow-up testing, and public health decisions.
Key Facts
- Lateral flow means liquid moves sideways through a porous strip by capillary action.
- Typical flow path: sample pad to conjugate pad to test line to control line to absorbent pad.
- Antibodies bind specific targets called antigens, such as viral proteins or bacterial markers.
- A visible test line forms when labeled antibody-target complexes are captured at the test region.
- A control line must appear to show that the liquid flowed correctly and the reagents worked.
- Sensitivity = true positives / (true positives + false negatives), and specificity = true negatives / (true negatives + false positives).
Vocabulary
- Lateral-flow test
- A rapid test that moves a liquid sample through a strip to detect a target molecule and show a visible result.
- Antigen
- A molecule, often from a virus, bacterium, or body process, that can be recognized by an antibody.
- Antibody
- A protein that binds to a specific target molecule with high selectivity.
- Test line
- The region of the strip where captured target molecules create a visible signal if the test is positive.
- Control line
- The region of the strip that should show a signal in a valid test to confirm proper flow and reagent function.
Common Mistakes to Avoid
- Reading the result too early or too late, which is wrong because the signal needs the specified time window to develop and can change after the allowed reading time.
- Ignoring the control line, which is wrong because a missing control line means the test is invalid even if a test line seems to appear.
- Assuming a negative rapid test always means no infection, which is wrong because low target levels, poor sampling, or early illness can produce a false negative.
- Confusing sensitivity with specificity, which is wrong because sensitivity measures how well positives are detected while specificity measures how well negatives are identified.
Practice Questions
- 1 A rapid test has sensitivity 90%. If 200 truly infected people take the test, how many false negatives are expected?
- 2 In a study, a rapid test correctly identifies 470 of 500 people who do not have the disease. What is the specificity as a percent?
- 3 A patient has symptoms but gets a negative rapid test result with a visible control line. Explain why a clinician might still order a laboratory PCR test or repeat the rapid test later.