title: IMx tacrolimus II assay: is it reliable at low blood concentrations? A comparison with tandem MS/MS.
abstract: OBJECTIVE
  To compare tacrolimus concentrations in blood samples using the MEIA II and MS methods at the lower and higher ends of the clinically relevant range.
  
  
  METHODS
  MEIA II was compared to our tandem MS/MS procedure by regression and difference plot analysis. Data from recently published CAP surveys comparing MEIA II with MS procedures are also analyzed.
  
  
  RESULTS
  Comparison of MEIA II with our tandem MS/MS procedure by regression analysis yielded r values of 0.612 and 0.829 for tacrolimus concentrations below and above 9.0 ng/mL respectively, as determined by MEIA II. Below 9 ng/mL between day imprecision of tacrolimus controls gave CVs of 12.16 for MEIA II and 7.82 for tandem MS/MS. Addition of known amounts of tacrolimus to EDTA whole blood gave percent target values of 148 and 130 at concentrations of 5.0 and 17.5 ng/mL respectively as determined by MEIA II. Difference plots demonstrated variability in the mean bias for MEIA II of 43% and 36% at the lower and higher concentration ranges respectively. Analysis of the CAP surveys suggested that the relative positive bias and inter-laboratory variability with MEIA II was more pronounced when the MEIA II median value was below 9.0 ng/mL.
  
  
  CONCLUSIONS
  Our results along with the CAP survey data suggest that tacrolimus concentrations below 9.0 ng/mL measured by MEIA II are questionable and should be interpreted with caution.
authors.affiliation: Department of Laboratory Medicine, Children's National Medical Center, 111 Michigan Avenue, NW, Washington, DC 20010-2970, USA.
authors.forename: Amit K
authors.initials: AK
authors.lastname: Ghoshal
authors.forename: Steven J
authors.initials: SJ
authors.lastname: Soldin
mesh_terms: D006801:Humans; D007124:Immunoenzyme Techniques / Q000379:methods*; D007166:Immunosuppressive Agents / Q000097:blood* / Q000737:chemistry; D013058:Mass Spectrometry / Q000379:methods*; D015203:Reproducibility of Results; D016559:Tacrolimus / Q000097:blood* / Q000737:chemistry
chemical_list: D007166:Immunosuppressive Agents; D016559:Tacrolimus