title: Describing Assay Precision-Reciprocal of Variance Is Correct, Not CV Percent: Its Use Should Significantly Improve Laboratory Performance.
abstract: BACKGROUND
  Describing assay error as percent coefficient of variation (CV%) fails as measurements approach zero. Results are censored if below some arbitrarily chosen lower limit of quantification (LLOQ). CV% gives incorrect weighting to data obtained by therapeutic drug monitoring, with incorrect parameter values in the resulting pharmacokinetic models, and incorrect dosage regimens for patient care.
  
  
  METHODS
  CV% was compared with the reciprocal of the variance (1/var) of each assay measurement. This method has not been considered by the laboratory community. A simple description of assay standard deviation (SD) as a polynomial function of the assay measurement over its working range was developed, the reciprocal of the assay variance determined, and its results compared with CV%.
  
  
  RESULTS
  CV% does not provide correct weighting of measured serum concentrations as required for optimal therapeutic drug monitoring. It does not permit optimally individualized models of the behavior of a drug in a patient, resulting in incorrect dosage regimens. The assay error polynomial described here, using 1/var, provides correct weighting of such data, all the way down to and including zero. There is no need to censor low results, and no need to set any arbitrary LLOQ.
  
  
  CONCLUSIONS
  Reciprocal of variance is the correct measure of assay precision and should replace CV%. The information is easily stored as an assay error polynomial. The laboratory can serve the medical community better. There is no longer any need for LLOQ, a significant improvement. Regulatory agencies should implement this more informed policy.
authors.affiliation: *Department of Pediatrics, University of Southern California Keck School of Medicine, Founder and Director Emeritus, Laboratory of Applied Pharmacokinetics, Children's Hospital of Los Angeles; †Department of Mathematics, University of Southern California; ‡Scientific Consultant, Laboratory of Applied Pharmacokinetics; §Department of Pathology, Biochemical Genetics, and Special Chemistry; and ¶Department of Pediatrics, Director, Laboratory of Applied Pharmacokinetics, Children's Hospital of Los Angeles, California.
authors.forename: Roger W
authors.initials: RW
authors.lastname: Jelliffe
authors.forename: Alan
authors.initials: A
authors.lastname: Schumitzky
authors.forename: David
authors.initials: D
authors.lastname: Bayard
authors.forename: Xiaowei
authors.initials: X
authors.lastname: Fu
authors.forename: Michael
authors.initials: M
authors.lastname: Neely
mesh_terms: D001774:Blood Chemical Analysis / Q000379:methods* / Q000592:standards*; D000068598:Data Accuracy; D016903:Drug Monitoring / Q000592:standards*; D006801:Humans; D057230:Limit of Detection; D015233:Models, Statistical