In the larger transplant centers where technical expertise is available, liquid chromatography with mass spectrometric detection (LC-MS) for tacrolimus therapeutic drug monitoring is replacing the popular microparticle enzyme-linked immunoassay (MEIA) as a cost-effective alternative technology. As more labs convert to LC-MS, the accuracy, precision, selectivity, and sensitivity of the tacrolimus MEIA are being challenged, using data from large populations of clinical samples. However, little attention has been paid to how the results of particular procedures may differ within and among individual patients and to how such differences may relate to patients’ characteristics or to relevant biochemical parameters. So, after validation of an LC/MS procedure and verification of an LC/MS/MS procedure, the author analyzed 552 serial trough blood tacrolimus samples, collected from 38 patients over a 3-month period, by controlled MEIA and LC-MS procedures. Corresponding hematocrit and serum albumin level data were obtained. In an attempt to investigate whether the observations of others who studied population-based data could be illustrated for individuals, longitudinal data from several patients were plotted to visually elucidate any relations between the tacrolimus concentration and biochemical data. Finally tacrolimus concentration, hematocrit, and serum albumin data were compared using data stratified by transplant type, in-/outpatient status, male/female gender, or period elapsed since transplant surgery. The validated/verified LC-MS procedures were shown to be much better controlled than the MEIA during the 3-month parallel comparison study period. The longitudinal data of several individuals who experienced wide changes in the biochemical parameters clearly illustrated the relation between the difference in tacrolimus concentrations determined by MEIA and LC-MS and hematocrit (and sometimes albumin). Differences between the MEIA- and LC-MS-determined tacrolimus concentration data were strongly correlated to transplant type, in-/outpatient status, gender, time elapsed since liver transplant surgery, hematocrit, and weakly to serum albumin levels. In summary, the LC-MS methods provide highly reliable and reproducible estimates of tacrolimus concentrations, whereas the performance of MEIA technology did not provide reliable long-term performance for longitudinal therapeutic drug monitoring of tacrolimus because it was effected by several inherent demographic factors and by factors that can change over time in transplant recipients.