Tacrolimus Assay Comparison: Analytical and Operational Considerations for Immunoassay vs Mass Spectrometry Testing...

Tacrolimus therapeutic drug monitoring is a routine and critical assay for managing transplant patients. Tacrolimus can be measured from whole blood samples via liquid chromatography-tandem mass spectrometry (LC-MS/MS) or immunoassay (IA). LC-MS/MS provides accurate parent drug quantification with minimal metabolite interference but requires expensive analyzers, highly trained staff, and is often a very manual process. Immunoassays offer faster, simpler workflows, but historically have been prone to significant cross-reaction with the drug's metabolites or other therapeutic drugs, leading to a falsely elevated result. Newer immunoassays show promising improvements in specificity.

Using 73 residual patient samples, we compared our in-house LC-MS/MS tacrolimus assay to the Roche Elecsys® tacrolimus electrochemiluminescence immunoassay on the cobas® e801 analyzer. Comparative analysis was done with Deming regression and Bland-Altman comparison. Linearity, within-individual imprecision, and total imprecision post immunoassay implementation were also evaluated, along with changes and improvements to workflow and turnaround times (TAT).

The Roche Elecsys tacrolimus assay correlated well with our LC-MS/MS method (Deming regression equation: y = 1.004x + 0.4848). The bias was 0.52 ng/mL (an average of 8.1%) as determined by Bland-Altman comparison. When switching to the immunoassay we observed a 42%-51% improvement in TAT across the median, 75th percentile, and 90th percentile.

We observed excellent correlation between the Roche Elecsys tacrolimus assay on an e801 analyzer vs our in-house LC-MS/MS assay, with good precision and linearity across reportable ranges. Changing from mass spectrometry to tacrolimus immunoassay substantially improved our TAT and workflow.