I've been trying to find CDC data on the current Covid-19 lab testing sensitivity and specificity but haven't been able to find anything. The closest thing I could find was this Medium article:
Bayes’ Rule, Decision Making, And Containing COVID-19 With Unreliable Diagnostic Tests.
Do tests even add any valuable information in Wuhan patients?
For a 40% sensitive, 90% specific test, we just saw that it takes 8 consecutive negative results to reduce a 50% probability of infection to less than 5% (where very low probabilities are required to reduce the risk of contagion). We also saw that it takes 2 consecutive negative results to reduce 8% to less than 5%. What is the probability of a person in Wuhan being infected, if they:
Have already shown signs in a CT scan? (surely >50% probability?)
Have a cough/fever? (surely >8% probability?)
Are related to someone who has been infected?
Have entered any hospital?
It seems like testing for the purposes of ruling out infection is not a feasible solution in an active outbreak zone. There are simply not enough resources to test everyone multiple times.
This may partially explain why…… Authorities have already implemented large-scale containment measures — they assume that everyone is, or will be infected.
I'm not sure how close his estimates on the sensitivity and specificity are (40% sensitivity is terrible), but he is spot on in his explanation of how the statistical reliability of any medical test factors into the diagnostic process (at least it's consistent with what I was taught). Even with tests that are both highly sensitive and specific you still wind up chasing down many false positives if they're not targeted toward the populations with higher probability or prevalence of disease (but good luck trying to explain that to another physician, let alone the soccer mom from Orange County). Or worse, as in the case of Covid19, if you wind up with test conditions that carry a very high probability of releasing false-negatives (people who actually are infected) into the general population so they can spread the novel virus to others.
His estimate of needing to perform 8 serial tests to get to a 95% confidence level for Covid19 confirmations kind of gives you an idea of the challenges facing public health systems with limited resources. And it also helps to explain why the leap to harsh methods like quarantine has some grim logic behind it, as well as why China decided to ease off the original case definition to include CT evidence of pneumonia without lab confirmation. Extraordinary circumstances call for extraordinary measures.
He's got another one:
The Reported Mortality Rate of Coronavirus Is Not ImportantLow Case-Fatality Rates Are Only Possible Because Of Modern Healthcare Services — What If We Lose These?
We know that case-fatality depends on access to healthcare services. This would be especially true for coronaviruses. But what would happen if the outbreak became too widespread, causing hospitals to run out of capacity and leaving people without access to healthcare services?
To see how we would fare without access to modern healthcare services, we look to the past. Spanish flu had no treatment at the time of outbreak in 1918, and it ended up causing at least 50 million deaths in the world with a case-fatality rate exceeding 2.5%. If SARS and MERS only achieved 10% and 34% with modern healthcare, then we could imagine it being far worse without it. Since the novel coronavirus produces similar symptoms and requires similar treatment, we may have a catastrophic situation if the outbreak strains healthcare resources, and thus, prevents additional patients from being admitted to hospitals.
The Availability of Resources — Where Is The Breaking Point?
Most of the figures we have now apply to Wuhan and Hubei. Hubei, the province of Wuhan, is economically in the top half of China’s provinces. Their healthcare system is decent, with 2.17 physicians and 5.46 hospital beds per 1000 people (from the 2015 China Health Statistics Year book). These numbers have probably grown since then. Wuhan, a city of 11 million, is one of 15 “new tier-1” cities, and according to a local government report in 2014, Wuhan had 6.51 hospital beds and 3.08 doctors per 1,000 people. Was this enough?
I think he's probably right. If this new virus causes enough infections to overload our medical capacity to care for severe cases, we won't be seeing low single digit fatality percentages either. China's major cities probably beat many Western countries in terms of medical capacity, although rural medical capacity in many parts of the US is already at crisis levels, so we better hope we can keep the epidemic at bay in North America for a year or so and hope for a vaccine.
It really sucks having to make the big decisions on how to fight a growing epidemic when you don't fully understand it or have the proper tools developed to combat a new virus.
