What do my antigen (PCR) tests results mean?

Author: Dr Mark Ali MBBS BSc FRCS CTh, Medical Director

Positive antigen result

The sample you have given is positive for SARS-CoV-2 (the covid-19 virus). This means that covid-19 viral RNA was detected in you on the day you took the test. 

How accurate are the results?

Specificity concerns the false positive rate and is expressed as a percentage. The PCR test has a 100% specificity. This means that there is no cross-reactivity and no false positives. If you have a positive result, it is extremely likely to be true. 

What should I do now?

It is extremely important that you and your household follow the latest self-isolation guidance issued by the government. At the time of writing, this would be to stay at home for seven days from the start of the onset of your symptoms. After day 7, if you do not have a high temperature, you do not need to continue to self-isolate. If you continue to have a high temperature, you must self-isolate until your temperature returns to normal. 

All other household members should stay at home for 14 days from the onset of your symptoms. If any household members display symptoms, they need to stay at home for 7 days from when the symptoms appeared, regardless of what day they are on in the original 14 day isolation period. You can read more about this and self-isolation on the government website: https://www.gov.uk/government/publications/covid-19-stay-at-home-guidance

Some individuals choose to be re-tested following resolution of their symptoms to see if the virus has been cleared. To indicate virus clearance, many institutions recommend two negative samples taken 24 hours apart. 

From 14 days after the onset of your symptoms, you could take an antibody test to see if you have developed antibodies to covid-19. 

If you would like to discuss your results further or have any concerns, please contact the clinic at: +44 (0)20 3916 5990.

If you feel unwell

If you need further advice, feel short of breath or have concerns you are not coping with the illness, please contact your GP or 111 via their website (https://111.nhs.uk/). Alternatively you may call them on 111. If you feel extremely unwell, please call 999.

Negative antigen result

The sample you have given is negative for SARS-CoV-2 (the covid-19 virus). This means that covid-19 viral RNA was not detected in your nose and throat on the day you took the test.

How accurate is the test?

The sensitivity of the test refers to the false negative rate and is expressed as a percentage. The PCR test for antigens has a minimum sensitivity of 98% if performed correctly. This means that, if performed correctly, out of every 100 people who have virus antigen on the swab, 2 people will falsely be given a negative result. 

What do my results mean?

A negative PCR result could mean that you are not currently infected by the virus. As you can see from the sensitivity, if the swab is performed correctly, it is highly likely that you do not carry the virus. However, the swab is difficult to perform and a poor quality sample reduces the sensitivity and therefore likelihood of false negative result. Alternatively, the virus may not be present at the site the sample was taken from. Timing may also be an issue: it could be too early or too late in the infection to detect replicating virus.

As you can see, it is important that results are considered within a clinical context. Taking into account the factors above, the false negative rate in some studies is up to 30%. This is why many institutions require repeat sampling at least 24hrs later in order to confirm a negative result. 

Risks of a false negative include: delayed or lack of supportive treatment, lack of monitoring of infected individuals and their household or other close contacts for symptoms resulting in increased risk of spread of COVID-19 within the community, or other unintended adverse events.

If your test is negative but you think there is a strong possibility you have/had symptoms of covid-19, it is essential that you continue to follow government guidelines for self-isolation of you and your household. You can read more about this and self-isolation on the government website: https://www.gov.uk/government/publications/covid-19-stay-at-home-guidance.

If you would like to discuss your results further or have any concerns, please contact the clinic at: +44 (0)20 3916 5990.