Rapid Antigen testing aims to see if the virus is detectable now. The test is taken to:
- Diagnose infection
- Test following infection to see if the virus has been cleared
- Screen asymptomatic individuals to detect if they carry virus and are potentially infectious
Whilst rapid antigen testing is not as sensitive as PCR methods which use laboratory testing, it is useful as part of a holistic approach to COVID management. Rapid antigen testing should be complimented by other strategies such as risk assessment, hygiene and awareness training, routine and regular hand washing, distancing, face coverings and employee monitoring.
Antigen tests are therefore a crucial part of mitigating transmission as we come out of lockdown. We need the ability to detect the carriage of covid-19 and respond to it rapidly. As people return to work and society tentatively re-opens, robust testing programmes as part of a return to work strategy will be key to reduce spread and improve confidence amongst employees and customers.
Test accuracy refers to two separate concepts: sensitivity and specificity.
Sensitivity concerns the false negative rate and is expressed as a percentage: out of 100 truly positive patients, how many are detected as positive by the test? The higher the sensitivity, the fewer the numbers of false negatives. In the USA, sensitivity is often called positive percent agreement (PPA).
Specificity concerns the false positive rate and is expressed as a percentage: out of 100 truly negative patients, how many are detected as negative by the test? The higher the specificity, the fewer the numbers of false positives. An example here would be if the test detects an antibody response to a different but related coronavirus. The individual would be informed that they have antibodies to covid-19 when in fact it would reflect infection with a different but similar coronavirus. In the USA, specificity is often called negative percent agreement (NPA).
Test results need to interpreted in context, the likelihood of a true positive or true negative will vary depending on the prevalence or how common the condition is within a population.
It is important to know when to take an antigen test, the highest, peak viral load is around the time of symptom onset. If using the test to diagnose, it is sensible to do an antigen test as soon as possible following symptom onset. Ideally the test should be performed within days 1-5 from onset of symptoms. Virus may be detected after this (and in some cases can be detected for a long time after) but the chances of detection decrease as the body clears the virus or it moves further down the respiratory tract (which requires a different sampling method).
What does a positive antigen test result mean?
A positive antigen test means that the covid-19 virus has been detected on the swab. These tests are highly specific, which means that the false positive rate is almost negligible. This means that the false positive rate is almost negligible. If the result is positive, it is highly likely to be true.
The test detects viral RNA and does not tell the difference between active viral infection and “dead” viral remnants. It therefore cannot distinguish between someone who is infectious (active virus) and someone who is not (dead virus shedding). To do this, you would need to extract the virus particles and try to infect cells to see if this is possible.
What does a negative antigen test result mean?
A negative result could mean one of several different things:
- A genuine negative test result: the person does not carry covid-19. They may never have had it or they have had it and cleared the virus to levels low enough that it is no longer detected.
- A false negative result: if the sensitivity of the test is 93.3%. This means that out of every 100 people who take the swab, 7 are incorrectly identified as negative when in fact they carry the virus. The technique used for the swab is therefore important. Nasopharyngeal swabs should be uncomfortable to take in order to get a good sample. Test results should always be evaluated within the clinical context. If there is a high index of clinical suspicion, the test should be repeated.
- The test is sampling the wrong part of the respiratory tract. The distribution of the virus across the respiratory tract varies between patients and over the course of the infection. Even if a person is infected, the virus may only be detectable in sputum or nasopharyngeal swab but not necessarily both locations at the same time.
What does the future look like?
Point-of-care (POC) testing that can be self-administered and results available within 20 minutes, is already here and this is what is currently offered. Ideally, the test would be non-invasive and the results not too dependent on correct technique used.
Holistic approaches and workplace strategies that combine training, risk assessment and point of care antigen testing will reduce transmission, drive down the “R” (reproductive number) and improve safety in the workplace.
For employers trying to get employees back to work, reassuring them and their customers, these tests, software such as COVID Gateway and training will truly enable a safer return to work strategy.