Free Covid tests end at an East Cambridge location Wednesday. (Photo: Banc d’Imatges Infermeres via Flickr)

Cambridge reported a startling 104 new cases of Covid on Wednesday, but it turns out that most of them were not new. Instead, health department staff performing a year-end “historical case review” discovered 80 older cases that hadn’t been included in the city’s Covid-19 data, Cambridge Public Health Department spokesperson Dawn Baxter said.

The remaining 24 cases were recent, Baxter said. The case review was “part of our year-end record-keeping,” she said.

“Most cases are reported within a few days of a positive test, but there are always some stragglers,” Baxter said. “Our median time from test to report date for the last month is two days.”

Every weekday the Cambridge Covid-19 data center posts the number of new Covid cases and deaths, based on reports of new cases that the city health department gets from a state Department of Public Health database. (Weekend figures are included in the Monday report).

At the same time, another table in the data center reports cases by date, based on when a resident first tested positive, had symptoms or some other event attached to the case. Those figures can change over time as the health department digs into case information and gets more details.

The case numbers by date may disagree with the daily tally of new reported cases. For example, the city posted 36 newly reported cases on Dec. 6, but there were 24 new cases on Dec. 6 based on the date of diagnosis or other case event.

Baxter said the case numbers by date were “more accurate” as an indicator of new cases.

The year-end historical case review also uncovered an additional coronavirus death, bringing the total to 180 deaths. The resident, who was living in long-term care, died in August, according to a statement on the data center website.

As Covid enters a new year, the city has accepted a new phase in detection: a move to using home tests. As a result, the city will stop offering free PCR tests as of Wednesday at CIC Health’s location at 296 Third St. Demand has dropped, said the city’s chief public health officer, Derrick Neal. The city has signed a new contract with the private company that will enable mobile testing and vaccination clinics, Neal said.