Half of Moscow’s coronavirus patients in serious condition have tested negative for the virus, Mayor Sergei Sobyanin said Thursday amid questions over the accuracy of Russia’s tests.

His revelation follows a directive for Russian doctors to diagnose Covid-19 without tests after a widely reported study placed their accuracy at around 70%. Moscow’s health department reportedly warned the city’s clinics in April that many coronavirus tests return false negative results.

“Up to 50% of seriously ill patients, many of whom are on ventilators and in intensive care, had two negative tests for coronavirus,” Sobyanin said.

“But we see that they’re seriously ill and this is definitely coronavirus,” the mayor said in an interview with the state-run Rossia 24 news channel.

Anna Popova, the head of the federal consumer protection watchdog Rospotrebnadzor, maintained Friday that Russia’s Covid-19 tests are “highly sensitive.”

“Our Russian test systems don’t miss anything,” the state-run TASS news agency quoted Popova as saying.

Russia has reported 114,431 coronavirus cases — making it the eighth most-affected country in the world — and 1,169 deaths as of Friday. Moscow accounts for about half of Russia’s Covid-19 infections with 57,300 officially registered cases.