A NY Times commentary from an infectious disease expert provides a great summary of where we are in coronavirus testing and why we are having so many problems getting more universal testing in place.
Summary of the points from the commentary:
1. It is important to distinguish between viral test, or RT-PCR tests, that directly measure viral infection and antibody tests, or serology tests, that measure antibodies against the virus and indicate whether you have previously been infected or not.
2. Both test types have their problems and are not perfect.
3. The viral tests are very specific, meaning if it is positive you are positive, but have high false negative rates, with reports that as many as 15 to 40 percent of those infected would test negative (my commentary: the false negative rate likely varies depending on how long you have been infected). This means even universal testing with these tests would not be enough to be sure we know who all is infected.
4. The antibody tests are not necessarily testing active infection, but also indicate previous exposure to the virus. This MIGHT, as with other viruses, indicate immunity from reinfection, but this is not known yet (my commentary: thus, talk of a "passport" or "certificate" of immunity is premature and not yet justified by a positive antibody test).
5. Current studies of antibody tests have had major flaws, including selection bias (i.e. done in areas of high outbreak or super spread events, or tending to accrue people who have reason to believe they may be infected) and false positive tests (more precisely, non-specific binding to other non-SARS-Cov-2 coronaviruses). (my commentary: in spite of that it is clear there are many asymptomatic infected and given those with mild symptoms haven't been tested, official incidence rates are extreme undercount by maybe factor of 10, but probably not 50-85 as suggested by Santa Clara study).
6. Antibody tests don't just give a yes/no answer, but a number that indicates levels of antibodies in blood (called antibody titer), and this number varies over time in different infected people as well.
7. Tracking the history of testing in USA, there was an outcry in the initial failure of CDC to develop reliable viral tests, followed by an opening up to private tests with far reduced scrutiny by the FDA, leading to numerous different tests out there (>61 viral tests, >136 antibody tests) for which we don't know which are good and which are not. They argue next step is for FDA to get involved and assess which work and recommend those.
8. There is unprecedented global demand for essential testing components leading to shortages in USA and around the world. This problem is not going to go away -- many countries would like to test universally but there is not enough supply to pull it off right now.
9. There needs to be an international cooperative effort to overcome this shortage and vastly increase production of supplies and tests.
10. We need to improve contact tracing.
11. We need to use "syndromic surveillance", or recording of telltale symptoms and communicating to officials, to track outbreaks while testing is inadequate.