SPRINGFIELD, Ill. (WCIA) — The way Illinois measures the spread of COVID-19 completely changed over the weekend.

The Illinois Department of Public Health went from tracking case positivity to case rate as of Monday morning.

A change in reporting guidance from the federal Centers for Disease Control and Prevention forced the change. The updated guidance no longer requires labs to report negative antigen tests.

Case positivity was calculated as the percentage of positive PCR, antigen and other tests out of the total number of tests conducted on a given day. Without labs submitting that total test number, which includes the negative antigen tests, it’s no longer possible for the state to calculate test positivity.

Instead, the rate will be measured by the percentage of the population that tests positive. IDPH announced the shift in a press conference Tuesday afternoon. Medical Director Dr. Arti Barnes said the case rate has followed a similar trajectory as case positivity throughout the pandemic.

The state, like much of the U.S., has seen a rise in coronavirus cases over the last two weeks. The positivity rate (that the state no longer tracks) was 2.3% on Friday. Friday’s case rate (that will be used going forward) was 12.1%.

The case rate is up to 13.7% of the state’s population as of Tuesday.