There’s been a lot of talk about testing folk for COVID-19. It would seem blindingly obvious testing is important. It’s not just about people knowing their status so they can get the right care and reduce the possibility of their infecting others, it’s also crucial to inform an evidence-based response to the pandemic by the authorities.
It also allows critical workers like those in healthcare to know if it’s safe to go back to work.
But as ever with biology, it’s complicated. I don’t want to get into the politics of mass testing; for this post I just want to do the nerdy bit about how the different tests actually work. It’s wankingly cool so put the kettle on, pull up a chair and let’s do some biochemistry porn…
There’s a number of methods available to diagnose COVID-19 but each has its own strengths and weaknesses.
This stands for Polymerase Chain Reaction. This is where you get a nose or throat swab and see if you can find the viral genetic material (RNA) in it. The PCR bit amplifies the signal by (usually) converting the RNA into DNA and then copying it many times until there’s enough to actually do the test.
Without getting into the fine detail of all the different PCR techniques now available it’s the same process used to amplify DNA recovered from crime scenes – and its inventor, Kary Mullis, got a Nobel for it.
But even though PCR exhibits both sensitivity and specificity it has its drawbacks; in the early stage of the disease the viral load might be too low to be detected or the subject might not have have major respiratory symptoms – and so little detectable virus in the nose and throat. You also need really good technique; it’s a very sensitive assay and samples are easily contaminated.
So, in many respects PCR is the ‘gold standard’ but it’s also time consuming, finicky and expensive.
This is the test that’s being rolled out now. It involves looking for evidence of the body’s reaction to the virus found in blood serum (or lymph or other extracellular fluid) – and it just needs a finger-prick of blood, something that looks like a pregnancy piss-stick and about 10 minutes. No labs. No mucking about.
Here’s how it works.
When our bodies are challenged by an invader, the body can mount both specific and non-specific defences. I want to concentrate on one specific response for now, antibodies.
An antibody or immunoglobulin (Ig) is a large protein used by the immune system to neutralise pathogens like bacteria and viruses. Individual antibodies are specific to a particular challenge and there are tons of them, one for each specific antigen you’ve ever been exposed to.
Antibodies recognise a molecule unique to each different pathogen (an antigen) and lock on to it. It’s like they recognise the fingerprint of a known offender. By attaching it tags a microbe or an infected cell for attack by other parts of the immune system. In some cases antibodies can neutralise the target directly by binding to a part of it that it needs to do its stuff and thus blocking it.
One of the first antibodies to be produced in an infection is IgM. It appears early in the course of an infection, then disappears but can reappear after subsequent exposure to the antibody if the person is reinfected.
The other class of antibody of interest here is IgG. It’s the most common type of antibody found in the body and does lots of cool stuff – but what’s of interest here is it stays in the blood and extracellular fluids after the infection has passed and after both IgM and any viral RNA have long gone.
You can see what I mean here:
So, viral RNA (from virus particles) is there from Day 1 of infection and if you can find it there’s a very specific test for it. Three days in IgM turns up and (in the case of COVID-19) is detectable until the end if the third week.
IgG doesn’t arrive until the end of Week 1 – but after that it’s around for good. It’s only in small – but detectable – quantities: it’s one of the parts of the immune system that’s keeping a watching brief for future infection. If the IgG comes across the specific antigen it was raised against it alerts the other wings of the immune system and starts a new response – and rapidly.