COVID-19-related hospitalizations in Italy: an application of the synthetic control method to investigate the trajectory of diagnosis-related group 79 and its counterfactual during 2010-2021.

Background: With 10.95 million cases (11 March 2020-9 February 2022), Italy was massively hit by the coronavirus disease 2019 (COVID-19) pandemic. Most of the COVID-19-related inpatient discharges were codified under the Diagnosis-Related Group (DRG) 79. During 2019-2021, DRG 79 inpatient discharges increased from 20,377 to 130,580 (+540.82%).

Research design and methods: To investigate the causal relationship between DRG 79 inpatient discharges and COVID-19, the synthetic control method (SCM) compared the real with the counterfactual DRG 79. The latter was a weighted combination of control units (22 DRGs unrelated to COVID-19). The SCM mimicked the trajectory of DRG 79 in the absence of COVID-19.Placebo studies and robustness test investigated the reliability of the baseline findings.

Results: Six out of the 22 control units contribute to the counterfactual DRG 79. The real and the counterfactual DRG 79 cease to overlap from 2019 onward. Placebo studies and robustness test confirm the causal relationship of COVID-19 with the increased number of inpatient discharges coded under DRG 79 during 2019-2021.

Conclusion: The SCM identifies a causal link between COVID-19 and DRG 79 in Italy. Hopefully, future contributions will utilize SCM (and causal inference in general) in health care decision-making within the Italian National Health Service.