Use of sample hematocrit value to correct blood tacrolimus concentration derived by microparticle enzyme immunoassay

The quality of microparticle enzyme immunoassay (MEIA) for blood tacrolimus is guaranteed in samples with hematocrit (Ht) values of 25 to 45%. Because MEIA provides inaccurate blood tacrolimus concentrations in samples with Ht out of this range (i.e. <25% or >45%), correction of the calibration is required for therapeutic drug monitoring. The authors demonstrated previously that overestimated MEIA tacrolimus concentration could be corrected by modified, calibrated MEIA (cMEIA) using the original calibrator. Here, an equation was established to more easily derive a corrected tacrolimus concentration by calculation (MEIAcalc) using the Ht of each sample. The tacrolimus concentrations of 99 whole-blood samples with low Ht (<25%) were then tested by the 3 assay methods: MEIA, cMEIA, and MEIAcalc. MEIA gave a significantly higher blood concentration of tacrolimus (median 12.9 ng/ml, range 3.6-26.4 ng/ml) than did cMEIA (median 10.0 ng/ml, range 0.2-21.1 ng/ml, p<0.05). This overestimation was eliminated by using MEIAcalc. There was no difference in blood tacrolimus concentration between cMEIA and MEIAcalc (median 10.0 ng/ml, range 1.7-21.4 ng/ml). MEIAcalc can be used to correct the tacrolimus concentration in samples obtained from patients with unstable Ht values.