The state hit a new record Wednesday for positive coronavirus cases. 17,227 diagnostic tests, 4,878 new cases and 88 new deaths were reported by the Arizona department of Health Services on their publicly available dashboard. The biggest question that inevitably is asked when it comes to the numbers: did these cases and deaths all occur in the past 24 hours?
The answer is mostly no.
COVID-19 data that is reported by the state health department comes from three main sources: laboratories; for case and test data, county health departments; for deaths and hospitalizations, and hospitals themselves; for bed counts and the number of COVID-19 patients being treated. Each of these sources has a different lag time that ranges from almost instantaneously to about 7-10 days.
Out of the three, hospital systems report the fastest. They provide the department with a so-called census, or the total count of beds both available and occupied. They do this for “inpatient” or hospital general purpose hospital beds, Intensive Care Unit and emergency department beds, and ventilators. They also report “COVID-19 specific metrics” that include COVID-19 patients utilizing inpatient and ICU beds, ventilators, as well as the number of emergency room visits, intubations and discharges.
This data is updated daily. Wednesday, Arizona had a reporting high of 89% ICU total occupancy, but a slightly lower number of COVID-19 patients than what was reported the day before.
Test collection data from laboratories are often the second fastest to report. Test data that is reported is typically a combination of negative and positive tests from the past seven days.
The charts below show what this lag looks like. The “Day Reported” chart shows case data as it is reported by state health officials, while the “Day of Positive Test” shows the actual date that the positive test occurred. The yellow line is a 7-day moving average used to smooth out the data to account for weekends.
County health departments, tasked with the investigative process of confirming hospitalizations and deaths that are attributable to COVID-19, are the slowest to report data. Deaths from COVID-19 reported by the state may have occurred anywhere from one to fourteen days prior, and sometimes that time span is longer. The chart below shows COVID-19 deaths both as they are reported and on the actual date of occurrence. The two charts differ from each other considerably; today has the highest reported number of deaths, but the highest date of deaths, at ttime of publication, is tied at 33 on June 15 and 16.
Because hospitalizations and deaths are slower to fill in, casual observers of AZDHS data may conclude that hospitalizations and deaths are going down. However, the health department warns against this on their dashboard by telling users that, “recent deaths may not be reported yet.”
As processes at the state level shift to respond to the COVID-19 challenge, some of these lags may change over time. It is very unlikely, due to the nature of the data collection surrounding COVID-19, that the state will ever be able to report what occurred in the past 24 hours.