The Effect of COVID-19 Pandemic Restrictions on Laboratory Monitoring of Lithium Treated Outpatients in the Netherlands: A Controlled Interrupted Time Series Analysis

Introduction: The COVID-19 pandemic restricted healthcare access. Mental illness in combination with isolation and fear of the virus possibly decreased routine monitoring for lithium using patients during the lockdown. Our aim was to study if monitoring frequencies and serum level values for lithium, TSH (thyroid stimulating hormone), and renal function changed during the COVID-19 pandemic. Methods: Using the PHARMO database (laboratory, pharmacy, and hospitalization data), we identified lithium users in The Netherlands over 2 years (14 October 2018–14 October 2020). The first year served as the control period; the second year was divided into pre-COVID, lockdown, and post-lockdown segments. A time series analysis with a linear regression model was performed to test for differences in monitoring frequency and aberrant serum levels at the beginning of the lockdown (immediate effect) and during the lockdown (post-lockdown trend, effect over 12 weeks). Results: We identified 2835 patients using lithium. Monitoring measurements declined by 52% (7.74% vs. 4.01%) for lithium serum levels, 28% (5.56% vs. 4.20%) for TSH, and 26% (17.7% vs. 13.2%) for renal function in the first week of lockdown. These reductions were statistically significant compared to the control period. Monitoring for all measurements gradually recovered during lockdown. Changes in aberrant serum levels were not statistically different during the exposure and control periods. Conclusion: Monitoring for lithium, TSH and renal function declined at the beginning of lockdown. However, it is unlikely that postponed laboratory measurements had clinically relevant negative treatment effects since no differences in aberrant serum levels were identified.