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Kidney Week

Abstract: PO2576

Significant Variability in Results from Different Tacrolimus Assays Is a Potential Recipe for Toxicity and Kidney Allograft Rejection

Session Information

Category: Transplantation

  • 1902 Transplantation: Clinical


  • Khan, Adnan A., University of California San Diego, La Jolla, California, United States
  • Smith, Jennifer C., University of California San Diego, La Jolla, California, United States

Tacrolimus (tac) is an immunosuppressive medication used to prevent organ rejection and prolong graft survival after transplantation. It is the main pillar of any combination immunosuppression regimen. Due to its narrow therapeutic window; it requires timely administration, close monitoring with 12/24 hours trough levels and dose adjustments. Previously done studies have shown a discordance between different assays, with a positive bias for immunoassay (2.8 -9.2%) compared to LC/MS-MS assay; due to cross-reactivity with tac metabolites.


Results from 33 stable kidney transplant patients who had routine transplant labs drawn at Labcorp® were reviewed. Each of these patients had tac trough levels checked by both LC/MS-MS assay and QMS immunoassay(Thermo-Fischer). The comparison of two assays was done with Deming regression and a Bland-Altman analysis for bias done using XLSTAT®. Linear regression was done using SPSS v26.


Deming regression analysis shows that there is a statistically significant difference between the two assays with y=1.36x-0.26. Bias(avg.) calculated by Bland-Altman plot was 2.35 ng/mL(Figure). Percentage difference in tacrolimus immunoassay and LC/MS-MS correlated with tacrolimus dose (p = 0.031).


A significant discordance of tac troughs (upto 85%) was found on our analysis; obtained by the two assays. This is clinically significant as tac has a narrow therapeutic window and adjustments of dose based on these assay results; can risk immunologic injury, DSA formation, rejection and nephrotoxicity at either ends of the spectrum. Standardization of assays in future, would be ideal. In the meantime, nephrologists should be adjusting trough goals based on the assay used, especially when different assays are in play.


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