Serum lithium as a compliance marker for food and supplement intake.

BACKGROUND Analyzing 24-h urine for lithium after consumption of lithium-tagged foods or supplements provides a validated compliance marker but is laborious. OBJECTIVE Most studies involve blood sampling; therefore, we tested whether serum lithium concentration could be used as a compliance marker. DESIGN We used serum lithium as a compliance marker in a dietary trial and an evaluation study. RESULTS In the dietary trial, 78 volunteers consumed 500 mL yogurt tagged with lithium (250 micromol/d) for 6 wk. Serum lithium increased from 0.9+/-0.3 to 6.6+/-1.5 micromol/L, which was close to the predicted concentration, indicating that the subjects were highly compliant. However, the interindividual variability in serum lithium concentration was large. To test whether this variability resulted from compliance differences or natural variability, we performed an evaluation study: 12 subjects took a lithium supplement (250 micromol/d) for 13 d under supervision. Serum lithium increased from 0.14+/-0.03 to 3.9+/-0.8 micromol/L (range: 2.6-5.4 micromol/L); thus, there was wide interindividual variation in serum lithium despite 100% compliance. However, within-subject variability was small, with a CV of 7% for serum lithium measured on 4 different days. We checked whether taking half the dose on each of 2 d (125 micromol lithium/d) would significantly lower serum lithium. Indeed, serum lithium dropped in all subjects, by a mean of 1.0 micromol/L on the first day (P<0.0001) and by another 0.3 micromol/L on the second day (P = 0.0004). Thus, changes in serum lithium concentration of > or =1.0 micromol/L suggest altered compliance. CONCLUSION Serum lithium concentrations after intake of lithium-tagged foods or supplements can be used to assess compliance in dietary trials.