Clinically Reviewed by Dr. Andrew Stein MD, Consultant Physician. Last updated: April 2026
Navigating COVID-19 testing is straightforward once you know what to look for. A lateral flow test (LFT) is a rapid diagnostic tool designed to provide results within 15 to 30 minutes by detecting specific viral antigens.
A positive result is indicated by the appearance of two lines on the test device.
The “C” Line: A colored line appears next to the “C” (Control) mark, showing the test worked.
The “T” Line: A colored line appears next to the “T” (Test) mark.
It is crucial to note that any visible line next to the “T”—no matter how faint—indicates a positive result. If both lines are present, the test has detected the SARS-CoV-2 virus.
A negative result is shown by a single line next to the “C” mark and no line next to the “T” mark. This suggests the test did not detect the virus at that time. However, a negative result is not a 100% guarantee that you do not have the virus, as viral levels can change throughout the day.
A test is void if no line appears next to the “C” mark. The control line is a built-in safety check to prove the liquid flowed correctly across the strip. If the “C” line is missing, the result is unreliable, and you must perform a new test with a fresh kit.
To ensure an accurate reading, always follow the specific manufacturer instructions included in your kit.
Preparation: Wash your hands and clear a flat surface.
Swabbing: Swab your nose (and sometimes throat) as directed.
Processing: Mix the swab in the liquid, then place the required drops into the test well.
Timing: Wait the exact time specified (usually 15–30 minutes) before reading.
If you test positive, you are likely infectious. Current guidance typically recommends:
Self-Isolation: Follow your local health protocols to prevent spreading the virus.
Further Testing: Depending on your location, you may be advised to book a laboratory-based PCR test for formal confirmation.
PCR tests are more sensitive than LFTs and are processed in specialized labs. In the UK, these can be scheduled via the Gov.UK website or by calling 119. In the US, testing sites can be located via the CDC or local health department websites.
The primary advantage of the LFT is its ability to identify infectious individuals quickly, even if they show no symptoms. While not as sensitive as a lab test, their speed makes them an excellent tool for regular screening and immediate peace of mind.
LFTs are highly reliable at identifying people with high viral loads who are most likely to transmit the virus. While false positives are very rare, they can occasionally occur due to user error or damaged kits.
We have descrbed a positive COVID lateral flow test (LFT) looks like:
Two Lines (C and T): Positive (even if one is faint).
One Line (C only): Negative.
No Line at C: Void (test failed; repeat with a new kit).
United Kingdom
NHS: How to do a COVID-19 lateral flow test – Official medical guidance on testing procedures.
GOV.UK: Understanding your test results – Government protocol for positive and negative results.
United States
CDC: COVID-19 Testing Overview – The primary US resource for self-testing and result interpretation.
FDA: At-Home OTC Antigen Tests – Information on test accuracy and authorised brands.
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