The BinaxNOW has emergency clearance from the F.D.A., however solely when administered within seven days after the onset of symptoms. Researchers don’t but know the way the take a look at will carry out in folks outdoors this window. Mr. Trump obtained a number of of those checks throughout his second week of sickness, all of which returned detrimental outcomes.

“The checks have for use after they’re supposed for use,” stated Andrea Prinzi, a scientific microbiologist on the College of Colorado, Anschutz. “That’s after they’re going that can assist you probably the most.”

Each P.C.R. and antigen checks have one other limitation: They search for hunks of the virus’s anatomy — particles from the pathogen — and can’t decide whether or not the virus remains to be lively. Some individuals who have been contaminated by the coronavirus might register as P.C.R. optimistic for weeks, even months, after they are no longer contagious or ill, just because checks are choosing up on innocent souvenirs of an an infection gone.

“Neither is a measure of precise, stay virus,” Dr. Garner stated, of P.C.R. and antigen checks.

That’s the place viral tradition is available in. Scientists can take a pattern from an individual’s airway after which attempt to coax the coronavirus into infecting cells in a lab. However these procedures aren’t broadly accessible to the general public, as a result of they must be carried out in a high-containment facility by folks particularly skilled to work with lethal pathogens.

These restrictions have led a number of scientists to experiment with P.C.R.-based checks that search particularly for subgenomic RNA as a possible proxy for active virus. Subgenomic RNA is a sort of genetic materials that’s produced solely after the coronavirus has contaminated a human cell. The compound can thus act as a form of molecular beacon that may alert researchers to a virus that has began to breed itself, with out the necessity to develop the damaging pathogen.

Dr. Conley’s memos about Mr. Trump appear to indicate that the president is not producing samples with detectable subgenomic RNA. Researchers have additionally tried to tradition the coronavirus outdoors of his physique, though few particulars on this have been shared. Virus that may be cultured isn’t essentially transmittable, or vice versa.

Neither viral tradition nor subgenomic RNA checks are permitted for widespread use, Dr. Garner famous. Researchers doing these research may not carry out their experiments in the identical approach, making any outcomes troublesome to interpret with out extra data how, and by whom, they had been obtained.

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