So much ink has been spilled and vitriol expended over the US’ incompetent testing regime … no surprise, our fearless leader has been extensively quoted saying things like testing only makes his numbers look bad (“If we did very little testing, we wouldn’t have the most cases. So, in a way, by doing all of this testing, we make ourselves look bad")
Much sturm und drang has also been spent on the CDC’s botched release of the first round of lab tests and the weeks spent fixing those problems … botched due amateur hour sterilization mistakes in production … what is less well known is that the CDC is not a production operation … they are super specialized advisers and coordinators as it relates to testing – they and not in the business of making test kits … why they were making test kits in the first place is another later and sadder post, but it revolves around the US having rejected WHO test kits
Lets flash forward to the kinds of testing available today … there is lot of hope that these tests will allow the “immune” to go back to work/shop/recreate etc… There are many concerns around this assumption … I’m going to focus on two – false positivity/negativity rates and Immunoglobulin Type
First the tests
PCR tests – these tests check for viral RNA fragments in the blood … they are the remnant by-product of ACTIVE infection and battles going on between phagocytes, leukocytes and the SARS-CoV-2 … your non-specific immune system is just chopping up invaders and disgorging the pieces … these viral pieces and by-products make it into your excretory systems … the nasal swab up the nose gets the mucus that contains this stuff … your kidneys filter it out and not many folks want to provide stool samples on demand ;-) … FYI - the virus has been found in semen and spit … breast milk is naturally filtered and safe ... A PCR test is only good until about two weeks after infection ... after that, you're not going to have enough viral RNA fragments for the test to pick up in a commercial nucleotide amplification process
Antibody Tests – If you recall from a past post, your body makes about 5 types of immunoglobulins (IgM, IgG, etc…) … these identify and latch onto external proteins on the SARS-CoV-2 surface … IgM is typically the first antibody/immunoglobulin produced and tags the invader for the phagocytes and leukocytes to kill … IgM is produced anywhere from 3-7 days post infection … after this first immune response is tapering down, the body starts producing IgG – think of it as memory antibody … this is much more specific, faster acting, and when it attaches to an invader, triggers a much stronger immune response … IgG appears about 3 weeks after infection … you want IgG antibody testing but it only makes sense to take this test 3-4 weeks after infection … much of what was available earlier this year was IgM testing … only recently do we have commercially available IgG testing
False Positive/Negative rates
With a virus that has infected 5-10% of the population, a false positive rate of 3% means that as many as 30-60% of those walking around thinking that they’re immune, ARE NOT (3/10 – 3/5) … this is a recipe for a guaranteed second wave of infection … the same math applies to the false negative rates … NPR reported the rapid Abbott test has a false positive of 15-20%!!!! and a lower but still significant false negative rate … I worked for years in highly regulated national Diagnostic testing, and I understand the desire for a rapid test - especially in a pandemic … Still, if the NPR figures are correct, I cannot fathom why this test was allowed to be sold given its weakness in signal detection and lack of accuracy … With the prevalence of SARS-CoV-2 infection, I’d look for a test with a false positive of less than 1% before hanging my hat, the state and national economy on it
Stay careful out there … minimize your exposure, wear masks, wash hands and be sensible about testing