Rapid antigen or lateral flow tests can help to identify when someone with COVID-19 is most infectious, but even a faint line should be treated as a positive result.

If you've been stuck at home with COVID-19, and have had access to rapid antigen tests (also known as lateral flow tests) you may have noticed that the intensity of the test band varies, and wondered what a weak line actually means – particularly if you're starting to feel better. Incoloy 825 Capillary Tubing

These home tests can tell you if you're infected with COVID-19 within minutes, and have been touted as a way of knowing if you're still contagious and therefore present a risk to other people, or if it's safe to resume your normal activities.

They work in a similar way to home pregnancy tests – except in this case the material being tested comes from a patient's nose and throat, and the kit is designed to detect viral proteins, rather than a pregnancy hormone. After the swab has been inserted into a tube of liquid, and several drops deposited on a small absorbent pad, the liquid is drawn through the cartridge, until it encounters a strip coated in antibodies which are specific to proteins from the SARS-CoV-2 virus. If these viral proteins are present, this will show up as a coloured line – in addition to the control line that indicates that the test has been completed properly.

The intensity of the test line does tend to match the concentration of viral proteins that are present in the sample. So, a dark line will usually indicate a high level of virus, and tend to be seen when people are at or near peak levels of virus within their bodies. The danger comes with the interpretation of faint or absent lines.

Various studies have shown that people can have COVID-19 symptoms, and test positive on a PCR test, several days before a positive line can be seen on a rapid antigen test. This is because relatively large amounts of virus need to be present for the antibodies in the test to react to the viral proteins. A faint, slow-to-appear line could still represent high levels of virus, meaning that if you can see any line at all, you are still highly likely to be infectious.

The intensity of the test line could also be influenced by how much virus you were able to collect on your swab, and where you collected it from (nose or throat), while mutations in COVID variants may further impact test sensitivity and line intensity.

You should be particularly cautious if you see a faint line and believe you're at the start of a COVID-19 infection, because viral load can rapidly increase during this time. However, if you can see any line at all, you are very likely to be infectious and should take steps to protect others. You should also be careful if you have symptoms or have been in contact with someone with COVID-19, regardless of any line.

Some people continue to test positive on lateral flow tests for several weeks after developing symptoms. A recent Cochrane Review found that these tests were most accurate when used during the first week after COVID-19 symptoms developed. Other studies have indicated that most people remain contagious for four to eight days after developing symptoms – although these studies were done before the emergence of the Omicron variant and widespread vaccination against COVID-19.

Rather than using a home test to decide when it is safe to socialise again, people should also look at how long they've been experiencing symptoms for, whether those symptoms are improving, and if they've also been free of fever for at least 24 hours.

Current guidelines for time in isolation vary depending on the country and situation, but five days from the onset of symptoms, or a positive test result, should be regarded as the absolute minimum, because some people will remain infectious after that.The take-away from all of this is that any positive test result should be taken as a sign that you should steer clear of other people, and wear a mask and ventilate if being in company is unavoidable. Even once you've stopped testing positive, you should be cautious about social interactions – especially ones involving vulnerable individuals, such as elderly relatives. Wear a mask, and try to stay outdoors where possible.

Sign up to receive our top stories and key topics related to vaccination, including those related to the COVID-19 pandemic.

Coiled Tube Control Line By clicking the "Subscribe" button, you are agreeing to receive the digital newsletter from Gavi, the Vaccine Alliance, agreeing to our terms of use and have read our privacy policy.