Suitability of chemiluminescent enzyme immunoassay for the measurement of blood tacrolimus concentrations in rheumatoid arthritis

The aim of this study was to evaluate the suitability of chemiluminescent enzyme immunoassay (CLIA) for the monitoring of whole-blood tacrolimus concentrations in rheumatoid arthritis (RA) patients. Sixty-three RA patients and 47 renal transplant (RT) patients treated with tacrolimus were enrolled. Tacrolimus concentrations in spiked blood and patient blood were measured by CLIA and HPLC-MS/MS. The cross-reactivity in CLIA was evaluated using 13- O-demethylated or 31- O-demethylated tacrolimus. Tacrolimus concentrations measured by CLIA correlated with those measured by HPLC-MS/MS. Bland–Altman analysis revealed the 95% confidence intervals between CLIA and HPLC-MS/MS in RA and RT patients were − 20.7 to 109.9% and − 5.0 to 74.1%, respectively. While 31- O-demethylated tacrolimus cross-reaction amounted to an equivalent of 120% tacrolimus in CLIA, 13- O-demethylated tacrolimus did not cross-react. CLIA values should be carefully interpreted in RA patients, especially those receiving a low dose of tacrolimus.