As rapid coronavirus tests are becoming more widely available, delivering results in minutes for patients in doctor's offices, nursing homes, schools and even the White House, officials warn of a significant undercount, blurring the virus's spread nationally and in communities where such tests are more commonly used.

Public health officials say that antigen tests, which are faster than polymerase chain reaction (P.C.R.) tests but less able to detect low levels of the virus, are an important tool for limiting the spread of the coronavirus. But they caution that with inconsistent public reporting, the case undercount may worsen as more “point-of-care” antigen tests, as well as D.I.Y. and home test kits, come on the market.

“We want to be sure that we’re not now saying, ‘there’s no disease,’ when there is lots of disease. All that’s happened is that the science with which we identify it has evolved,” said Janet Hamilton, the executive director of the Council of State and Territorial Epidemiologists, the group that helps the Centers for Disease Control and Prevention define cases of the coronavirus....