Context: Tacrolimus is an immunosuppressant that undergoes therapeutic drug monitoring. The laboratory at our institution changed its immunoassay techniques from the fluorescence polarization immunoassay to the cloned enzyme donation immunoassay.
Objective: To evaluate the relationship between the 2 assays and to determine the impact of the change on clinical decision making.
Design: A retrospective study of patients admitted to the hospital during the assay transition period. Tacrolimus values for the 2 assays were collected for 4 weeks and compared.
Setting: An academic health center.
Patients: Liver transplant patients hospitalized from February 18, 2008, to March 18, 2008.
Main outcome measure: The primary outcome was the agreement between the results of the 2 immunoassays. Secondary outcome was agreement of clinical decision making with established patient-specific therapeutic ranges or with a 30% difference in absolute values between the assays.
Results: Seventy-nine pairs of tacrolimus concentrations were collected from 21 liver transplant patients. The mean (SD) tacrolimus concentrations were 7.36 (4.21) microg/L for the fluorescence polarization immunoassay and 9.00 (5.30) microg/L for the cloned enzyme donation immunoassay (P = .03). A clinically different decision would have been made if the fluorescence polarization immunoassay value had not been reported 51% of the time. A Bland-Altman plot indicated no relationship between the assay results.
Conclusion: A change in tacrolimus monitoring assay would have resulted in different clinical decisions 51% of the time. Awareness of changes in assay technology must be heightened to enhance clinical decision making and prevent potential impact on morbidity among liver transplant patients.