Re: Covid 19: NEJM and former CDC director launch stinging attacks on US response
Responding to Bernadette Pajer and Janet Menage.
Clarifying the statement provided by FDA in the SARS CoV2 PCR method document.
"Since no quantified virus isolates of the 2019-nCoV are currently available, assays designed for detection of the 2019-nCoV RNA were tested with characterized stocks of in vitro transcribed full length RNA (N gene; GenBank accession: MN908947.2) of known titer (RNA copies/μL) spiked into a diluent consisting of a suspension of human A549 cells and viral transport medium (VTM) to mimic clinical specimen”
One essential step in method characterization is to determine the limit of detection (LoD). As this requires a quantitative readout from the assay, one needs to use a precisely quantified reference substance as calibrant, ie it needs to be known in the reference that the SARS CoV2 RNA genome is exactly present at a concentration of X copies/µl.
This statement simply means that this exactly quantified isolate is not availabe, but the isolate are available. The exactly quantified reference substance is available in the form of in-vitro transcribed RNA. This was used for the quantitative studies to establish the LoD.
This statement does NOT mean that no isolates are available. The CDC distributes currently 19 different SARS CoV2 isolates.
A very simple analogy:
apples = SARS CoV2 isolates
Do you have apples available?
Do you have buckets of exactly 50 apples available?
But you do have apples in store?
Yes, we do and we sell them in buckets, but some buckets contain as little as 40 apples and some may contain up to 60 apples.
Competing interests: No competing interests