No band appears, neither in “C” nor in “T”: the control has not worked well, then the test is invalid. We cannot conclude any results, neither negative nor positive.
Green band in “C” and nothing in “T”: the control works well and no virus antigens are detected, negative result.
Green band in “C” and red band in “T”: the control works well and virus antigens have been detected, positive result.
The green band does not appear in “C”, but the red band does appear in “T”: the control has not worked well, therefore the test is also invalid. Nor can we conclude any result, neither negative nor positive, even if the red band has appeared in “T”.
How to interpret a negative result
If it has been negative, does it mean that we are not infected? No. In most cases, from the beginning of the infection, there are a few days (four or five) in which the amount of virus in the sample may be very small and insufficient for the antigen test to detect.
The limit of detection of antigens is higher than the limit of detection of RNA by PCR: a higher viral load is required for the antigen test to detect the virus. Therefore, it can happen that the antigen test is negative but that you are in the first days of the infection, and even highly contagious.
In other words, a negative test is not a guarantee of not being infected and of not being able to infect others.
How to interpret a positive result
If we have tested positive, what does it mean and what should we do? It means, with a very high probability, that we are infected with the virus. Our viral load is high enough to be detected by the test. In addition, we will be in the most contagious phase, and we can transmit the disease very easily even if we do not present symptoms. We should isolate ourselves immediately, notify our health center to confirm the result by means of a PCR (a much more sensitive test) and notify our contacts.
As we have just seen, the advantages of the antigen test are its speed and simplicity. It does not require expensive reagents, machines or highly qualified technical personnel, and it is much cheaper than PCR. To top it all, you can do it yourself at home.
Although rapid antigen tests have been the subject of some controversy due to their low sensitivity compared to PCR, there is a growing body of data demonstrating their enormous potential to control the pandemic.
Why can antigen tests help control the pandemic?
The detection of SARS-CoV-2 is essential for the control of the disease and has been based mainly on quantitative RT-PCR (polymerase chain reaction, with reverse transcriptase).
This PCR detects the genome of the virus from nasal or pharyngeal samples and amplifies it in 30 to 40 cycles, which allows to identify even a minimal number of copies of the virus RNA. PCR is a very powerful clinical test, especially when a patient is or was recently infected with SARS-CoV-2. The virus’s RNA fragments can remain for weeks after the infectious virus has been shed, often in people without symptoms.
However, for public health measures another approach is needed. The objective is not to know if someone has RNA from the virus in their nose, perhaps from a previous infection, but if they are infectious at this particular moment.
Most people infected with SARS-CoV-2 are contagious for 4 to 8 days. Specimens generally do not contain potentially contagious virus (culture positive) beyond day 9 after symptoms onset, and most transmission occurs before day 5. This time is consistent with observed patterns of transmission of the virus (usually from two days before to 5-7 days after the onset of symptoms). That’s where the recommendation of 10 days of isolation comes from.
This transmissibility window contrasts with a mean of 22–33 days in which CRP can be positive (longer with severe infections and somewhat shorter among asymptomatic individuals). This suggests that 50-75% of the time an individual is PCR positive they are likely to be post-infectious (traces of the virus RNA are detected but the virus is not active).
PCR amplifies, the antigen test does not
Once the immune system has controlled the replication of SARS-CoV-2, the levels of PCR-detectable RNA in respiratory secretions drop to very low levels when individuals are much less likely to infect others. The remaining RNA copies can take weeks, even occasionally months, to disappear, during which time the PCR remains positive.
It therefore makes sense that a test such as antigen has a sensitivity of 30-40% compared to PCR when testing a random sample of asymptomatic people.
Although antigen tests have a lower analytical sensitivity than PCR (they require more virus to test positive), their ability to detect individuals with high viral load and therefore contagious is as high as CRP. And its specificity, that is, the ability to correctly identify those who are not infected, is comparable to PCR.