Sunday, 14 September 2014

The proportion of fatal cases (PFC)...

This is excerpted and altered a little, from a more influenza A(H7N9) virus slanted article to be found here. But I think it deserves its own page.

In July 2013 I coined a term on VDU to avoid the use of the term Case Fatality Ratio/Rate/Risk (CFR). 

My term was the Proportion of Fatal Cases (PFC). I use the term on VDU and have published it an article.[1] I have no expectations that anyone else will use it although I notice it made an appearance in a HeathMap story[2] by Maia Majumder.

The PFC is a percentage calculated as the currently known number of fatalities divided by the number of total lab-confirmed cases including fatalities, regardless of whether surviving cases are inpatients (hospitalized) or outpatients.

The PFC is just a number - it's not meant to imply that every case that ever happened is included - it never could. It does not account for those cases who will die later on, either directly or indirectly, as a result of their infection but who may be alive at the time of calculation. 

The PFC is a snapshot to be used before an outbreak is done and dusted. It is meant as a guide to what is happening right now using the data we can get our hands on. Sometimes that means lots of data and sometimes they are very limited or just plain behind closed doors.

The CFR makes use of the number of recovered cases in its denominator.[3] So it's important to know survivor numbers. As suggested above, this requires that all the people who will recover from their infection, have recovered (and been discharged) from their infection. 

Using the CFR early in an emerging virus/disease outbreak, when what usually brings in outbreak to our attention is death, is great for selling papers, but not helpful realistic in a bigger picture sense. 

The CFR is most useful at the end of an epidemic/pandemic, but not so much when data-in-hand is poor during the early days of many outbreak. 

Of course, some will take a PFC and multiply it by the world's population as an estimate of how many are going to die if the virus reaches pandemic levels. That's not helpful or accurate. Just accept it as that snapshot of what's happening now.

References...
  1. J. P. Dudley and I. M. Mackay. Age-Specific and Sex-Specific Morbidity and Mortality from Avian Influenza A(H7N9). J. Clin. Virol. 2013. Nov;58(3):568-70. ePub Sept.
    http://www.ncbi.nlm.nih.gov/pubmed/24091087
  2. http://www.healthmap.org/site/diseasedaily/article/estimating-fatality-2014-west-african-ebola-outbreak-91014
  3. http://en.wikipedia.org/wiki/Case_fatality_rate