title: The impact of calcium assay change on a local adjusted calcium equation.
abstract: BACKGROUND
  Deriving and validating local adjusted calcium equations is important for ensuring appropriate calcium status classification. We investigated the impact on our local adjusted calcium equation of a change in calcium method by the manufacturer from cresolphthalein complexone to NM-BAPTA.
  
  
  METHODS
  Calcium and albumin results from general practice requests were extracted from the Laboratory Information Management system for a three-month period. Results for which there was evidence of disturbance in calcium homeostasis were excluded leaving 13,482 sets of results for analysis. The adjusted calcium equation was derived following least squares regression analysis of total calcium on albumin and normalized to the mean calcium concentration of the data-set. The revised equation (NM-BAPTA calcium method) was compared with the previous equation (cresolphthalein complexone calcium method).
  
  
  RESULTS
  The switch in calcium assay resulted in a small change in the adjusted calcium equation but was not considered to be clinically significant. The calcium reference interval differed from that proposed by Pathology Harmony in the UK.
  
  
  CONCLUSIONS
  Local adjusted calcium equations should be re-assessed following changes in the calcium method. A locally derived reference interval may differ from the consensus harmonized reference interval.
authors.affiliation: Department of Clinical Biochemistry & Metabolic Medicine, Liverpool Clinical Laboratories, Royal Liverpool & Broadgreen University Hospitals NHS Trust, Liverpool, UK sarah.davies@rlbuht.nhs.uk.
authors.forename: Sarah L
authors.initials: SL
authors.lastname: Davies
authors.affiliation: Department of Clinical Biochemistry & Metabolic Medicine, Liverpool Clinical Laboratories, Royal Liverpool & Broadgreen University Hospitals NHS Trust, Liverpool, UK.
authors.forename: Charlotte
authors.initials: C
authors.lastname: Hill
authors.affiliation: Department of Clinical Biochemistry & Metabolic Medicine, Liverpool Clinical Laboratories, Royal Liverpool & Broadgreen University Hospitals NHS Trust, Liverpool, UK.
authors.forename: Lisa M
authors.initials: LM
authors.lastname: Bailey
authors.affiliation: Department of Clinical Biochemistry & Metabolic Medicine, Liverpool Clinical Laboratories, Royal Liverpool & Broadgreen University Hospitals NHS Trust, Liverpool, UK.
authors.forename: Andrew S
authors.initials: AS
authors.lastname: Davison
authors.affiliation: Department of Clinical Biochemistry & Metabolic Medicine, Liverpool Clinical Laboratories, Royal Liverpool & Broadgreen University Hospitals NHS Trust, Liverpool, UK.
authors.forename: Anna M
authors.initials: AM
authors.lastname: Milan
mesh_terms: D002118:Calcium / Q000032:analysis*; D006706:Homeostasis; D006801:Humans
chemical_list: D002118:Calcium
keywords: Adjusted calcium; equation; pathology harmonization