Over the course of the current Covid-19 crisis, the importance of reliable, accessible testing to screen for the disease has become increasingly apparent. At the moment, there are essentially 2 types of testing option – PCR swab to test for current infection and the antibody test to check for previous infection. Both are not perfect and have their plus and minuses.
Diagnostic or PCR test
What is it for? Doctors use this test to diagnose people who are currently sick with COVID-19. This is offered free in the hospital if you have symptoms and at Covid-19 assessment centres organised by the council/government. It can also be purchased through private clinics/organisations to be delivered to your home.
How does it work? This test uses a sample of mucus taken from a person’s nose and throat. It looks for the genetic material of the coronavirus, known as RNA. The test uses a technology called PCR (polymerase chain reaction), which greatly amplifies the viral genetic material if it is present. That material is detectable when a person is actively infected.
How accurate is it? Generally speaking, these are the most reliable tests. However, if the virus stops shedding (i.e. replicating) after a few days of symptoms, the test won’t identify someone who has recently been infected. Swabs can also sometimes fail to pick up signs of active infection (for example – the swab is placed in the wrong chamber of the nose or if the virus has not started shedding yet). Genetic material can also be present when someone does not have symptoms so they may miss being diagnosed as a silent carrier.
It’s worth noting that PCR tests can be very labour intensive, with several stages at which errors may occur between sampling and analysis. False negatives can occur up to 30% of the time with different PCR tests, meaning they’re more useful for confirming the presence of an infection than giving a patient the all-clear.
Warwick Medical School honorary clinical lecturer Dr James Gill said: “During the course of the outbreak, the PCR testing has been refined from the initial testing procedures and with the addition of greater automation to reduce errors. As such, we now have an 80-85% specificity – i.e. the chance the test is detecting the virus. Remember as we are looking at swabs taken from people, who have lots of other organisms floating around, we are essentially dealing with the question of how ‘right’ the result we are looking at is.”
How quick is it? These samples are generally sent to centralised labs for analysis, so it can take several days to come back (up to 10 days from one lab we found). Currently we are using The Doctor’s Laboratory which has a turnaround time of 2 days. Wait times were longer earlier in the pandemic because of a testing backlog.
PCR gives us a good indication of who is infected. They can be isolated and get in contact with people they’ve been in touch with so they can be quarantined too, just in case.
What is it for? This test mainly identifies people who have previously been infected with the coronavirus. This is primarily a good way to track the spread of the coronavirus through a population.
How does it work? This is a blood test from a finger needle-prick. It looks for antibodies to the coronavirus. Your body produces antibodies in response to an infectious agent such as a virus. Depending on which antibody the test is picking up, it will show whether the person is at the early onset stage of the infection or had past infection.
There are 2 main antibody types – IgM and IgG. IgM antibodies generally arise after 4 days to more than a week after infection so the test can detect early onset. IgG antibodies generally arise after a week or two, after which time the virus should have been cleared from the system. If there is only IgM, it indicates early infection, the patient is contagious or is a silent carrier, by which we recommend a follow on PCR swab to confirm the diagnosis. IgG indicates past infection, patient is no longer contagious and have antibodies for “memory” immunity.
How accurate is it? There are more than 120 antibody tests on the market. The Food and Drug Administration has allowed them to be marketed without FDA authorisation and quality is a great concern. A few tests have voluntarily submitted to extra FDA approval. Other tests are being validated by individual medical labs or university researchers. The antibody test kit we use are from BioMedomics and it is CE marked and has the Certificate of Conformity. It detects both IgM and IgG. It is also being used by Harvard Medical School and Massachusetts General Hospital to carry out research on Covid-19.
In general, these tests aren’t reliable enough to act based on the results alone. That’s why it’s important to have this test with a practitioner who can go through your history and exposure risk to help interpret the results. The BioMedomics antibody test has a sensitivity (percentage of people with the disease who are correctly identified by the test) is 88.66%. False negative rate is therefore 11.34% (i.e. the percentage of people who have the disease who were not detected by the test). However, recent studies by Harvard Medical School shows sensitivity increases after 7 days of symptoms and almost 100% after 12 days (for patients younger than 80 years old). Specificity of the antibody test (percentage of people without the disease who are correctly excluded by the test) is 90.63%. False positive rate is therefore 9.37% (compared to PCR which is as high as 30%).
How quick is it: These tests generally produce results in a few minutes, based on a drop of blood taken from the finger. Some research labs use a more sophisticated antibody test, called an Elisa (Enzyme-linked immunoassay) that are more accurate but are not as widely available.
How long does immunity last?
This is a million-dollar question. It’s not yet clear how long any immunity period after a Covid-19 infection will turn out to be. Historical studies have indicated that people who survived the 2003 sudden acute respiratory syndrome (SARS) outbreak had antibodies in their blood for years after recovery. Both SARS and Covid-19 are caused by coronaviruses, but it’s too early to say if Covid-19 will generate a similar immune response. Reports also indicate that some people have been infected with the virus twice over, meaning these particular patients didn’t develop any immunity at all.
The World Health Organization published a scientific briefing warning against governments who are now suggesting that the detection of antibodies to the virus “could serve as the basis for an ‘immunity passport’ or ‘risk-free certificate’ that would enable individuals to travel or to return to work assuming that they are protected against re-infection. There is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection,” according to the report.
For our patients who have a positive antibody test, we advise that it reduces their chances of reinfection but as the virus is still new and can potentially mutate, we recommend to stay vigilant, boost their immunity and continue preventative measures.
The more testing, the clearer the picture
The more testing we do, the clearer the picture is on who is infected and thus who needs to be isolated. Singapore, South Korea, Germany, they seem to have had a better course of the pandemic so far than other countries who don’t have such a high testing capability. South Korea for example, where tests for the disease were made promptly and readily available when the outbreak first hit, has had a drastically lower death rate than counties which have responded less promptly. Only 174 Covid-19 fatalities have been recorded in South Korea out of over 10,000 recorded cases, compared to 2,921 deaths in the UK out of nearly 34,000 recorded cases.
This can help alleviate some of the restrictions on movement sooner and give a better idea of what’s going on. It’s vital that testing, whether it’s PCR or antibodies, is ramped up as much as possible to provide clear evidence on what is happening and where we should be going.
PCR swab and antibody testing for Covid-19 is currently available in Dr Terry Clinic. If you would like to make an appointment please Contact Us.