title: Clinical performance characteristics of a new photometric lithium assay: a multicenter study.
abstract: BACKGROUND
  Therapeutic monitoring of lithium is important because of its narrow therapeutic range and therapeutic index, low protein binding and single route of elimination. We characterized a new photometric method that avoids the specialized requirements of ion-specific electrode (ISE), atomic absorption and flame emission methods.
  
  
  METHODS
  Minimum detectable concentration (MDC), linearity and calibration drift over 65 days were determined. Within-run, between-run and total imprecision were assessed over 20 days in accordance with NCCLS EP5. Interference studies were conducted for 46 endogenous and exogenous compounds. Two production lots of the new photometric method (LI) were compared on the Dimension(R) RxL system and two ISE methods [Ciba Corning (n=124) and DuPont (n=131)], an established photometric method (Vitros) 950 system; n=63) and atomic absorption (Thermo-Jerell Ash; n=63).
  
  
  RESULTS
  The MDC was 0.04 mmol/l. Linearity was demonstrated from 0.12 to 5.8 mmol/l by the regression equation: observed=(1.01 x expected)-0.0005 mmol/l, S(y/x)=0.03 mmol/l, r=0.999. Drift for the lithium calibrators over the 65-day study period was <5%, except for the lowest calibrator, which showed 0.04 mmol/l drift. None of the 46 potential interfering substances showed greater than a 6.5% difference between control and test solutions. ISE method comparisons showed the following: LI=(1.08 x Ciba Corning ISE)-0.15 mmol/l, S(y/x)=0.05, r=0.999, and LI=(1.03 x DuPont ISE)+0.00 mmol/l, S(y/x)=0.06 mmol/l, r=0.999. Comparison of the LI method with the atomic absorption and Vitros system showed proportionality error <10%. Bias between the LI method and atomic absorption was 7%, substantially less than that documented in proficiency surveys for the Vitros and other systems. No lot-to-lot or site-to-site differences were observed.
  
  
  CONCLUSIONS
  This new photometric method is an attractive alternative for Li measurement and is adaptable to instruments having spectrophotometric capability.
authors.affiliation: Department of Pathology, University of Maryland School of Medicine, Baltimore, MD 21201, USA. rchristenson@umm.edu
authors.forename: Robert H
authors.initials: RH
authors.lastname: Christenson
authors.forename: Joseph J
authors.initials: JJ
authors.lastname: Mandichak
authors.forename: Show Hong
authors.initials: SH
authors.lastname: Duh
authors.forename: Joan M
authors.initials: JM
authors.lastname: Augustyn
authors.forename: John C
authors.initials: JC
authors.lastname: Thompson
mesh_terms: D001667:Binding, Competitive; D002138:Calibration; D004396:Coloring Agents; D005562:Formazans; D006801:Humans; D007202:Indicators and Reagents; D008094:Lithium / Q000097:blood*; D015203:Reproducibility of Results; D013053:Spectrophotometry
chemical_list: D004396:Coloring Agents; D005562:Formazans; D007202:Indicators and Reagents; D008094:Lithium