https://nn.neurology.org/content/10/6/e200159
In many Long COVID clinics and studies, people need a history of a positive coronavirus test, but this clinic accepts people without one. The study found that many of the people with Long COVID symptoms had immunological evidence of prior coronavirus infection. They recommend that Long COVID clinics do more expansive immunological testing of patients, specifically N-specific T-cell and antibody responses, which tended to be the most robust.
In the first year of the pandemic tests were limited, so many people had suspected coronavirus infections, but were unable to be tested. Those with mild or asymptomatic infections might have not bothered getting tested. In addition, the tests have relatively high false negative rates, such that individuals who were actually infected tested negative. And to compound matters, people can turn seronegative on commercial antibody tests months after infection, such that antibody tests can't always confirm previous coronavirus infections.
This study suggests that millions may be dealing with Long COVID without a prior positive coronavirus test.
*PVS- postviral syndrome. NP- Neuro-PASC. HC- healthy controls.