How to make use of Antigen Tests for Covid-19? How do antigen tests differ from RT PCR?
This article aims to remove all the existing confusions on Covid-19 antigen tests. Also, explains the difference between the results obtained from antigen tests vs RT PCR tests.
Antigen tests target the spike protein on the surface of coronavirus and not the viral genome (RNA) inside as the RT PCR tests do.
Antigens won't be present before the infection or after the infection has passed. Hence, an antigen test can only reveal if a person is currently infected with SARS-CoV-2.
Accuracy of antigen tests:
Here it gets a little tricky. Positive results from antigen tests are highly accurate, but negative results could be false. What does this mean? It means, there is a high possibility of getting false negatives, and negative results do not rule out infection.
Antigen tests typically have high specificity (99.3%- 100%) and low sensitivity (50.6% to 84%)
This means that a positive result is 99.3% to 100% accurate (Not even a single negative/ healthy case will be picked as positive/ infected). You can trust a positive result.
By contrast, a negative result could be only 50.6% to 84% accurate (a positive/ infected case may be missed at least 16% times). In other words, a swab sample with too little antigen may be missed, leading to a false-negative result.
That's why the recommendation is to trust a positive result and need not follow it up with an RT PCR, whereas if a person is symptomatic and the rapid antigen test results come negative, then one should follow it up by an RT-PCR test to confirm the diagnosis.
That's why RT -PCR has to be repeated in negative cases, to check if the 'antigen-negative' result is 'truly negative'. If there was too little antigen that was missed by the antigen test, rest assured that it will be picked by the RT PCR. So the test was antigen-negative but RT PCR as positive. An RT-PCR test is considered very accurate and reliable because it can detect even a single virus particle. If RT PCR also comes negative then the test is truly negative.
An RT PCR can capture a specific gene from genetic material (viral RNA) in a swab sample, and multiply it through a series of reactions and confirm its presence. That's why RT PCR tests are time-consuming & expensive, whereas the antigen tests are rapid and cheap.
When confirming an antigen test result with an RT-PCR test, it is critical that the time interval between the two sample collections is less than two days, and there have not been any possibilities or events for new exposures between the two tests. If more than two days separate the two tests, or there have been occasions for new exposures between the two tests, the RT PCR test should be considered a separate test – not a confirmatory test.
Antigen Tests vs RT PCR (Interim guidelines by CDC.gov updated 16 Aug 2020)
When to use antigen tests in healthcare settings (ICMR guidelines)
ICMR has approved a point-of-care rapid antigen test for diagnosis of COVID-19, in addition to the already existing molecular diagnostic tools. Antigen test is a promising tool for quick diagnosis of COVID-19.
i) All symptomatic Influenza-Like Illness (ILI) patients presenting in a healthcare setting and are suspected of having COVID-19 infection. ILI case is defined as one with acute respiratory infection with fever >= 38*C and cough.
ii) Asymptomatic patients who are hospitalized or seeking hospitalization, in the following highrisk groups: Patients undergoing chemotherapy, Immunosuppressed patients including those who are HIV+; Patients diagnosed with malignant diseases; Transplant patients; Elderly patients (>65 yrs of age) with co-morbidities (lung disease, heart disease, liver disease, kidney disease, diabetes, neurological disorders, blood disorders)
iii) Asymptomatic patients undergoing aerosol-generating surgical / non-surgical interventions: Elective/emergency surgical procedures like neurosurgery, ENT surgery, dental procedures; Non-surgical interventions like bronchoscopy, upper GI endoscopy and dialysis.
Making use of Rapid Antigen Tests in dental clinic:
If a person gets his rapid antigen test done before a dental appointment, rapid and cheap, and the results come positive, don't take him for the treatment. Simple. If he is antigen-negative, he may still be infected for which he may require RT PCR confirmation. But infected cases that are antigen-negative have less viral load and may not be "infectious" to spread it to others. Also, with antigen tests, we will at least remove the positive "infectious" cases and prevent the spread.
Thus, we have to make use of the high specificity of Rapid Antigen Tests. Now, if we overcome the problem of "Low sensitivity" as well, then Rapid Antigen Tests will become the game-changers.
Overcoming the low sensitivity problem:
A 15-minute Covid antigen test from Abbott Laboratories has received emergency authorization for use in the U.S. The test, called the BinaxNOW COVID-19 Ag Card, costs just $5 and works without requiring any laboratory equipment. It uses a nasal swab and a small reactive card. Easy to use, it can be administered by a range of health-care workers, including pharmacists, at almost any location. Abbott Labs plans to manufacture 50 million tests a month by October.
Coming to the main part - accuracy. BinaxNow has 97.1% accuracy.
This can be a game-changer. Abbot also gives a free app called NAVICA, for iPhone and Android users. The app will allow people who test negative to show a “temporary digital health pass” marked with the date of your last test from their phones.
Currently approved antigen tests in India:
Pathocatch Covid-19 Antigen Rapid testing kit costs Rs. 450/- in India. It is developed and manufactured in India by Mylab Discovery Solutions. Earlier ICMR had approved Standard Q Covid-19 Ag Detection Assay (South Korean firm SD Biosensor).
References: All accessed on 27 Aug 2020