Context Dependency of Serum and Urinary Lithium: Implications for Measurement of Proximal Sodium Reabsorption
To the Editor:
As reported previously in this journal, the endogenous lithium clearance is a marker of proximal tubular sodium handling.1,2 To investigate the context dependency of the endogenous lithium clearance, we measured by electrothermal atomic absorption spectrophotometry the lithium concentrations in serum and in exactly timed urine samples in 745 whites (51.5% women) and 266 blacks (62.8%) who were recruited randomly from the population in Belgium1 and South Africa.3 Mean age (±SD) was 40.6±15.8 and 42.6±18.2 years, respectively. The average (±SD) concentration of lithium in tap water (1.37±0.56 versus 0.03±0.01 μmol/L) and the 24-hour urinary lithium excretion (8.2±5.6 versus 3.1±4.1 μmol per 24 hours) were higher and more dispersed in Belgium than in South Africa (P<0.0001 for differences in means and variances; Figure). These results strongly suggest that the environment, probably via the food chain,4 determines the dietary intake of lithium. By contrast, the serum lithium distributions (0.31±0.16 versus 0.32±0.21 μmol/L) were very similar in means (P=0.58) but not in variances (P<0.0001). These observations suggest that serum lithium is tightly regulated despite large variations in dietary intake. The rank correlations (±SE) between the lithium and creatinine clearances (0.44±0.04 versus 0.42±0.06; P=0.70 for between-country difference) and between the serum and urinary lithium concentrations (0.30±0.04 versus 0.22±0.06; P=0.26) were similar across countries. The latter observation suggests that the endogenous lithium clearance, whether expressed as clearance or as fractional excretion, remains a useful marker of proximal sodium reabsorption under various environmental conditions.
Sources of Funding
The study was financed in part by InGenious HyperCare (LSHM-CT-2006-037093), a grant of the European Union.
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