"If 80-90% of the people test positive, you are probably missing a lot of cases," says Michael Ryan, executive director of the WHO Health Emergencies Program.
That's because more positives means the tests are being used mainly to confirm obvious cases, and as a result health authorities are not getting a clear picture on the scope of the epidemic in their area.
In places where every close contact and suspected case has been tested, the percent of negative cases is high. Ryan says in countries that have extensive testing, fewer than 12% of their tests are positive.
"We would certainly like to see countries testing at the level of ten negative tests to one positive, as a general benchmark of a system that's doing enough testing to pick up all cases," Ryan says.