title: Analytical robustness of nine common assays: frequency of outliers and extreme differences identified by a large number of duplicate measurements.
abstract: BACKGROUND
  Duplicate measurements can be used to describe the performance and analytical robustness of assays and to identify outliers. We performed about 235,000 duplicate measurements of nine routinely measured quantities and evaluated the observed differences between the replicates to develop new markers for analytical performance and robustness.
  
  
  METHODS
  Catalytic activity concentrations of aspartate aminotransferase (AST), alanine aminotransferase (ALT), and concentrations of calcium, cholesterol, creatinine, C-reactive protein (CRP), lactate, triglycerides and thyroid-stimulating hormone (TSH) in 237,261 patient plasma samples were measured in replicates using routine methods. The performance of duplicate measurements was evaluated in scatterplots with a variable and symmetrical zone of acceptance (A-zone) around the equal line. Two quality markers were established: 1) AZ95: the width of an A-zone at which 95% of all duplicate measurements were within this zone; and 2) OPM (outliers per mille): the relative number of outliers if an A-zone width of 5% was applied.
  
  
  RESULTS
  The AZ95 ranges from 3.2% for calcium to 11.5% for CRP and the OPM from 5 (calcium) to 250 (creatinine). Calcium, TSH and cholesterol have an AZ95 of less than 5% and an OPM of less than 50.
  
  
  CONCLUSIONS
  Duplicate measurements of a large number of patient samples identify even low frequencies of extreme differences and thereof defined outliers. We suggest two additional quality markers, AZ95 and OPM, to complement description of assay performance and robustness. This approach can aid the selection process of measurement procedures in view of clinical needs.
authors.affiliation: Institute of Clinical Chemistry and Laboratory Medicine, University Medicine Greifswald, Greifswald, Germany.
authors.forename: Stefanie
authors.initials: S
authors.lastname: Neubig
authors.affiliation: Institute of Clinical Chemistry and Laboratory Medicine, University Medicine Greifswald, Greifswald, Germany.
authors.forename: Anne
authors.initials: A
authors.lastname: Grotevendt
authors.affiliation: Department of Clinical Chemistry, Karolinska University Hospital, Stockholm, Sweden.
authors.forename: Anders
authors.initials: A
authors.lastname: Kallner
authors.affiliation: Institute of Clinical Chemistry and Laboratory Medicine, University Medicine Greifswald, Greifswald, Germany.
authors.forename: Matthias
authors.initials: M
authors.lastname: Nauck
authors.affiliation: Institute of Clinical Chemistry and Laboratory Medicine, University Medicine Greifswald, Greifswald, Germany.
authors.forename: Astrid
authors.initials: A
authors.lastname: Petersmann
mesh_terms: D001681:Biological Assay / Q000592:standards*; D015415:Biomarkers / Q000097:blood*; D019411:Clinical Laboratory Techniques / Q000592:standards*; D003955:Diagnostic Tests, Routine / Q000592:standards*; D006801:Humans; D011785:Quality Assurance, Health Care; D011786:Quality Control; D012016:Reference Values
chemical_list: D015415:Biomarkers
keywords: analytical quality; clinical chemistry tests; humans; quality control; reproducibility of results