The mysterious double positives could simply be the result of human error. Hospitals have been testing for the presence of the virus using swab samples from a patient's nose, throat, and sometimes lungs. Swabs can yield different quantities of the virus depending on where the clinician swabs and how they do it, Lewin said. If the swab sample yields too little of the virus, it could lead to a false negative for a patient who is still infected.
Problems with the testing kits themselves can also lead to false negatives or false positives—like the
batch of faulty kits the U.S. Centers for Disease Control and Prevention rushed out and then ordered back, a blunder that
delayed testing in the U.S. by weeks and may have contributed to the virus's spread there.
The Wuhan man who died after he was discharged and readmitted had
twice tested negative for the virus before being released, though a pre-discharge CT scan indicated a remaining infection in his lungs.