Falsely Elevated Whole Blood Tacrolimus Concentrations due to Interference in an Affinity Column-Mediated Immunoassay Method on Xpand Dimension

Falsely Elevated Tacrolimus (FK506) Trough Levels in a Liver Transplant Recipient

Antibody-conjugated magnetic immunoassay (ACMIA) for tacrolimus (FK506) may detect falsely elevated tacrolimus trough levels, a commonly underreported event. We report a case of falsely elevated whole-blood tacrolimus levels in a patient post-orthotopic liver transplantation. A 71-year-old male patient underwent liver transplantation in 2012. Post-transplantation, the patient was immediately started on tacrolimus for maintenance immunosuppression. His most recent dose was 0.5 mg four times weekly. During monitoring, trough levels were at 25.9 ng/mL using ACMIA. After this result, a decision was made to hold tacrolimus. After holding tacrolimus for seven days, detected trough levels were still continually greater than 20 ng/mL. Upon suspicion of falsely elevated results, liquid chromatography with mass spectroscopy (LC-MS) was used to check tacrolimus trough levels. Results showed normal trough levels of 7.6 ng/mL. Because of its narrow therapeutic window, tacrolimus levels need to be carefully monitored throughout treatment. When high tacrolimus levels are detected using ACMIA without a correlating clinical scenario, trough levels should be re-confirmed using LC-MS to prevent clinical decisions from being made based on falsely elevated results.

False Elevation of the Blood Tacrolimus Concentration, as Assessed by an Affinity Column-mediated Immunoassay (ACMIA), Led to Acute T Cell-mediated Rejection after Kidney Transplantation

Tacrolimus is the most commonly used immunosuppressant. Because of its narrow therapeutic range, it is necessary to frequently monitor its concentration. We report the case of a 25-year-old man who underwent kidney transplantation whose tacrolimus concentrations, as measured by an affinity column-mediated immunoassay, were falsely elevated. As we reduced the dose of tacrolimus, the recipient developed T cell-mediated rejection. Using the same blood samples, an enzyme-multiplied immunoassay technique showed that the patient's levels of tacrolimus were extremely low. A further examination indicated that the false increase in the tacrolimus concentration was likely due to an unknown interfering substance. We administered methylprednisolone and antithymocyte-globulin. The patient's serum creatinine level decreased and remained stable after these treatments.

False tacrolimus concentrations measured by antibody-conjugated magnetic immunoassay in liver transplant patient: 2 case reports and literature review

Safe use of tacrolimus relies on regular whole-blood drug monitoring. Of the methods used to assess whole-blood tacrolimus concentration, antibody-conjugated magnetic immunoassay is mostly used for therapeutic drug monitoring because it requires only a minimal sample preparation and no pretreatment procedure. However, several cases recently have been reported in which abnormally false elevated tacrolimus concentrations were measured by antibody-conjugated magnetic immunoassay (>15 ng/mL), despite the absence of clinical symptoms. We present 2 cases of falsely detected tacrolimus concentrations that did not show abnormally high values within the therapeutic range. Whole-blood tacrolimus concentrations obtained by antibody-conjugated magnetic immunoassay showed well-controlled concentrations (approximately 2-8 ng/mL), whereas those obtained by another immunoassay and in washed erythrocytes were below the assay range (< 1.2 ng/mL). Thus, antibody-conjugated magnetic immunoassay can elicit falsely positive results of tacrolimus concentrations, even though they are within the therapeutic range.

Standardization of LC-MS for Therapeutic Drug Monitoring of Tacrolimus

BACKGROUND

LC-MS is increasingly used for therapeutic drug monitoring of tacrolimus. A recent summary from an international proficiency-testing scheme demonstrated that the mass spectrometry respondents were the largest method group. However, these methods lack standardization, which may explain the relatively poor interlaboratory agreement for such methods. This study aimed to provide one path toward the standardization of tacrolimus quantification by use of LC-MS.

METHODS

A 40-member whole blood tacrolimus proficiency panel was circulated to 7 laboratories, 4 in the US and 3 in Europe, offering routine LC-MS–based quantification of tacrolimus. All laboratories used a common LC-MS platform and followed the manufacturer's instructions that accompanied an LC-MS reagent kit intended for tacrolimus quantification in whole blood samples. Four patient pools were prepared that had sufficient volume to allow comparison with a tacrolimus reference measurement procedure.

RESULTS

For the 40-member panel, the standardized MassTrak LC-MS assay demonstrated excellent agreement with a validated LC-MS method used by Analytical Services International (y = 1.02x − 0.02; r = 0.99). The CVs for the pooled patient samples ranged from 2.0% to 5.4%. The mean difference from the reference measurement procedure ranged from 0.4% to 4.4%.

CONCLUSIONS

Tacrolimus assay standardization, which must include all facets of the analysis, is necessary to compare patient results between laboratories and to interpret consensus guidelines. LC-MS can provide accurate and precise measurement of tacrolimus between laboratories.