title: Evaluation of 25-hydroxy vitamin D assay on the immunodiagnostic systems iSYS analyser.
abstract: BACKGROUND
  We evaluated the recently released chemiluminescence assay for 25-hydroxy vitamin D (25-OHD) on the Immunodiagnostic Systems iSYS (IDS-iSYS) automated analyser.
  
  
  METHODS
  The IDS-iSYS comparison was performed using patient samples previously measured for 25-OHD by a validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) method (n = 119) and an IDS enzyme immunoassy (IDS-EIA) method (n = 64). Limit of detection and limit of quantification were determined from a precision profile. Imprecision was assessed using quality control material and pooled serum. External QAP material (Vitamin D External Quality Assessment Scheme, UK) was analysed to establish inaccuracy. Linearity was assessed by two dilution studies. Cross-reactivity was determined by three serial dilution studies of patient samples with known 25-OHD(2) concentrations.
  
  
  RESULTS
  The IDS-iSYS correlated well with both established methods (iSYS = 1.03LC-MS/MS - 6.53, R(2) = 0.82 and iSYS = 1.07IDS-EIA - 1.61, R(2) = 0.86). Imprecision of the iSYS assay for IDS control material was 13.4% at 32 nmol/L, 10% at 78 nmol/L, 9.4% at 161 nmol/L, and for the pooled material 9.3% at 72 nmol/L and 5.6% at 158 nmol/L. The evaluation found the assay to be highly accurate (IDS-iSYS = 0.93ALTM + 3.79, R(2) = 0.94) and linear (obs(1) = 0.93exp(1) - 5.05, R(2) = 0.99 (P = 0.256); and obs(2) = 0.97exp(2) + 6.07, R(2) = 0.97 (P = 0.654); ALTM, all-laboratory trimmed mean). Cross-reactivity studies demonstrated no significant difference to the calculated total 25-OHD as measured by LC-MS/MS.
  
  
  CONCLUSIONS
  Even though the imprecision of the iSYS was found to be greater than that of the LC-MS/MS and EIA methods, the performance characteristics of the IDS-iSYS 25-OHD assay are suitable for routine diagnostic purposes on a high throughput automated analyser.
authors.affiliation: Metabolic Laboratory, Chemical Pathology Directorate, SA Pathology, Box 14, Rundle Post Office, Adelaide SA 5000, Australia. zee.cluse@health.sa.gov.au
authors.forename: Zee N
authors.initials: ZN
authors.lastname: Cluse
authors.forename: Andrew N
authors.initials: AN
authors.lastname: Fudge
authors.forename: Malcolm J
authors.initials: MJ
authors.lastname: Whiting
authors.forename: Brett
authors.initials: B
authors.lastname: McWhinney
authors.forename: Ian
authors.initials: I
authors.lastname: Parkinson
authors.forename: Peter D
authors.initials: PD
authors.lastname: O'Loughlin
mesh_terms: D002853:Chromatography, Liquid / Q000379:methods*; D006801:Humans; D057230:Limit of Detection; D015203:Reproducibility of Results; D053719:Tandem Mass Spectrometry / Q000379:methods; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood
chemical_list: D014807:Vitamin D; C104450:25-hydroxyvitamin D