eLi12 represents a new method to estimate the serum-lithium (Se-Li) concentration at 12 hours after the most recent lithium dose. eLi12 is a mathematical equation that presents the clinician with an estimated 12-hour Se-Li level when the lithium blood test is not taken 12 hours after the most recent lithium dose, but e.g., after 8 or 16 hours. The eLi12 equation is based on a development study and supported by two independent proof-of-concept studies. In these studies, eLi12 estimated a Se-Li concentration that was significantly closer to the 12-hour Se-Li level compared to the actual measured Se-Li in all scenarios between 3 hours and 24 hours after the most recent lithium dose. Before eLi12 can be implemented into a clinical setting, it needs to be tested whether lithium-treated patients actually can register the time for the last lithium intake (prerequisite for eLi12) and if eLi12 can provide improvements for everyday clinical work. The objective of this study is to test 1) whether lithium-treated patients can register the time of the most recent lithium intake at the time of their lithium blood test (which is a necessary data point for the eLi12 equation), 2) the difference between eLi12 and Se-Li, 3) if eLi12 will have less variation compared to the actual measured Se-Li concentrations, 4) patient satisfaction with the eLi12 solution and self-reported symptoms, and 5) clinician satisfaction with the eLi12 solution.