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The concept of ‘gold standard’ is too narrow and imprecise. The term Covid-19 applies to a continuous process from its contraction, incubation process, symptomatic viraemic stage +/- complications to either recovery or death. Underlying this are the body’s immunological, reparative and homeostatic response that can be diagnosed independently and supported (eg ventilation for respiratory failure). 

There will be different  ‘gold standards’ for these different ‘stages’ and complications which actually merge into each other. These different stages can be confirmed by different ‘sufficient’ diagnostic criteria that are man-made (eg a positive PCR is considered a sufficient criterion for the viraemic phase). Any sufficient criterion can be a gold standard that confirms Covid-19 but the absence of any one alone cannot exclude. There can also be man-made ‘necessary’ criteria that have to occur so that their absence excludes by definition and convention. These concepts apply to all diagnoses and diseases of course. 

Huw

On 30 Mar 2020, at 22:50, Rod Jackson <[log in to unmask]> wrote:

 Hi James. You are correct that we need a gold standard to determine a false negative definitively. However comparing studies like the ones I quote is meaningful if they used similar criteria. I am sure the true false negative proportions in the studies I quoted will not be 0% and 80%, but they will be ball-park figures and highlight the key issue about their dependence on the pre-test probability. Moreover, they will change by the day depending on both testing proportions and disease prevalence. I didn’t bring up the other important issue which is the quality of the sample, which I understand can be quite variable.

Cheers Rod

On 31/03/2020, at 10:44 AM, McCormack, James <[log in to unmask]> wrote:

 Hi Rod - thanks for the positive comments - really appreciate it. So many people are doing great work around the evidence - and primarily around the lack of evidence.

Also agree with your comments about the false positives.

The false negatives are interesting - I was just wondering how can we define a false negative - to do that we have to have a gold standard for the diagnosis - at present that is PCR - but if the virus should clear over time how do we differentiate between a false negative and the fact that it is negative because the virus has cleared. Maybe I’m missing something.

James


On Mar 30, 2020, at 2:13 PM, Rod Jackson <[log in to unmask]> wrote:

Thanks very much James for sharing this. In New Zealand groups of academic public health medicine docs are starting to write and peer review rapid reviews related to Covid-19 and I have shared your post with our local co-ordinator.

I assume that similar groups around the world are doing similar things, so could I please strongly encourage list members to follow the example set by James and share these and also peer review them.

I just did a rapid critique of your review James and thought it was excellent.

James, on a related topic, In your last post you mentioned that the specificity of PCR was pretty much 100%. I discussed this with colleagues and we agreed that even if there are possibly a few false positives, we should assume there are none.

A much more complicated issue is the so-called ‘false negative rate’ (i.e. the proportion of all negative tests that are false), which one of my colleagues conducted a rapid review on. This is actually 1 minus the negative predictive value (NPV) and for the pedantic is a proportion, not a rate. But the key point is that, like the positive predictive value (PPV), it is highly dependent on the pretest probability (disease prevalence) in those tested. A study in asymptomatic French nationals who had been in Wuhan found a false negative proportion of 0, while another study of older Chinese people with CT evidence of viral pneumonia, found a false negative proportion of 80%. I thought this was a good reminder that the PPV and NPV are highly context specific. 

Cheers Rod Jackson
University of Auckland
New Zealand

On 31/03/2020, at 5:46 AM, McCormack, James <[log in to unmask]> wrote:

 Hi Everyone

Just posted our first COVID-19 Rapid Review - Tools for Practice. All about hydroxychloroquine. It will take you just a few minute to read and you will be up-to date as of now, but things are changing hourly as we all know.

Will have one soon on assessing dyspnea virtually.
James

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