Estimating the 12-Hour Serum Lithium Level (eLi12): Development and Two Proof-of-Concept Studies.

Objective: Most serum lithium (se-Li) tests do not comply with the required timing (12 hours after lithium intake). We aimed to develop an equation estimating 12-hour se-Li levels when lithium blood tests are taken at other time points than 12 hours.

Methods: The equation was developed via secondary analyses using data from the Bipolar Clinical Health Outcomes Initiative in Comparative Effectiveness (CHOICE) trial and verified in 2 separate proof-of-concept studies. Bipolar CHOICE included 122 lithium-treated patients (192 se-Li measurements), who had se-Li levels measured and self-reported the time since lithium intake. The proof-of concept studies tested the accuracy of the equation by measuring se-Li concentrations at different time points up to 24 hours after lithium intake and were performed in Boston, US (5 patients, 10 se-Li measurements) and Aarhus, Denmark (21 patients, 159 se-Li measurements).

Results: We present a simple equation calculating the estimated 12-hour se-Li level (eLi₁₂) based on the measured se-Li level at the patient-reported time for intake of the last lithium dose. The accuracy was confirmed in both proof-of concept studies, where eLi₁₂ showed a mean deviation from the 12-hour se-Li level of 10% compared to 25% for the measured se-Li (P < .0001). For 99 out of 102 (97%) blood tests taken between 3 and 24 hours after the last lithium dose, eLi₁₂ was closer to the 12-hour level than the actual measured se-Li level.

Conclusion: eLi₁₂ provides clinicians with more accurate 12-hour se-Li estimations and gives patients flexibility as to when to show up for lithium blood tests.