I’m going to keep sharing stuff that helps me, and helpfully you make sense of the coronavirus pandemic. I know that you all have a lot on your mind. So I will keep these short and sweet.
The currently available “diagnostic” tests for SARS-CoV-2 are NAAT – Nucleic Acid Amplication Tests, specifically, Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) tests that utilizes sections of the genetic code of the RNA virus as as a template that a reverse transcriptase amplifies into complementary DNA millions of times over, until a quantitative PCR can detect it.
Patients generally provide a sample of cells from the nasopharynx, via a delightful swab. It is imperative that the swab is done properly. The sensitivity and specificity of the current PCR tests for SARS-CoV-2 are unknown. Very preliminary data suggest that the sensitivity (SNOUT – rule out) could be as low as ~75-85%. The specificity is better ~98-100%. A major caveat is that a gold standard test is not available. CT is not the gold standard contrary to what early Chinese reports would tell us – especially in children.
And finally, there are existing viral panels / respiratory pathogen panels available at many hospitals that cost a lot of money and run PCRs for multiple viruses. The current PCR for “coronavirus” tests for four strains – just not SARS-CoV-2.
PCR tests take a day or two to come back, right?
Yes, it generally takes 24 to 72 hours for a PCR to result. Early on here in the United States PCR tests are in short supply. So it is absolutely critical that we explain the test’s characteristics to patients and families. I can absolutely see how they would confuse it with the rapid flu and strep tests with which they are familiar. This is why work towards molecular testing, which is less sensitive and specific, but faster is so important. We will accept a higher false negative rate in a pandemic as long as there is a confirmatory test (PCR) to back it up.
Ultimately, if molecular tests have a “good enough” positive predictive value they can be used to get a much quicker estimate of the burden on the public.
