Lithium determination in outpatient clinics by an ion-selective electrode in venous and capillary whole blood

The use of an ion-selective electrode (ISE) to determine lithium (Li) in routine clinical application was evaluated by repeatedly analyzing reference specimens (precision evaluation) and by comparing blood concentrations in Li-treated patients assessed by ISE and flame emission spectrometry (FES) (correlation and agreement). Precision evaluation was sufficiently high. Li values determined by ISE in venous and capillary whole blood showed high correlations with FES plasma values (correlation coefficients between 0.86 and 0.99). Within the therapeutic range (0.3-1.0 mmole/l Li), agreement was sufficient for venous and less satisfactory for capillary blood (mean differences, FES minus ISE: -0.03 and -0.11 mmole/l Li). Above the therapeutic range, ISE values markedly exceeded FES results.