Evaluation of an immunoassay of whole blood sirolimus in pediatric transplant patients in comparison with high-performance liquid chromatography/tandem mass spectrometry

: Sirolimus is widely used as an immunosuppressant, along with calcineurin inhibitors. Because of its variable pharmacokinetics and narrow therapeutic range, therapeutic drug monitoring of sirolimus is critical to optimize its therapeutic effect and to minimize toxicity. Although liquid chromatography/tandem mass spectrometry is considered the method of choice, the technical and financial challenges of this method are obstacles to its use. A microparticle enzyme immunoassay on the Abbott IMx has recently been reintroduced to the clinical diagnostic market. : We evaluated this immunoassay using high-performance liquid chromatography/tandem mass spectrometry as the reference method. Precision and carryover were evaluated using an expanded CLSI EP10-A2 protocol. Linearity was studied by serial dilution of high-level whole blood samples, and clinical utility was demonstrated by correlation with the reference method using 56 de-identified pediatric patient samples. : The total imprecision was less than 12% across the concentrations tested. The method was linear from 2.6 to 31nM. The immunoassay showed a mean positive bias of 11.5% in patient specimens relative to high-performance liquid chromatography/mass spectrometry (p