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.