The effect of paraproteins and rheumatoid factor on four commercial immunoassays for vancomycin: implications for laboratorians and other health care professionals.

BACKGROUND: Paraproteins, immunoglobulins (Igs), which are elevated in various autoimmune disorders, are known to interfere with various laboratory immunoassays, including vancomycin (VANC). Rheumatoid factor (RF), a known immunoassay interferant, may cause falsely elevated results. OBJECTIVES: The aims of this study were to (1) evaluate the effect of 3 paraproteins (IgA, IgG, and IgM) on 4 commercial VANC immunoassays [fluorescence polarization immunoassay; enzyme multiplied immunoassay; 2 particle-enhanced turbidimetric inhibition immunoassays]; (2) determine the concentration at which the effect is obtained, and (3) examine the influence of RF on the VANC methods. METHOD: Serum and plasma pools from patients prescribed VANC and a spiked VANC pool (20 mg/L) were each mixed 1:1 with individual patient specimens containing IgA (6-63 g/L), IgG (6-54 g/L), IgM (3-30 g/L) (n = 4 for each Ig), and a patient RF pool (196 IU/L). The mixtures (n = 39) were split and distributed for VANC analysis. RESULTS: IgA and IgG in serum and plasma did not affect any of the VANC immunoassays. RF added to plasma specimens did not interfere, but in serum, elevated VAN results were observed. IgM did not affect the fluorescence polarization immunoassay and enzyme multiplied immunoassay methods but did attenuate VANC concentrations by both particle-enhanced turbidimetric inhibition immunoassays (Siemens, Beckman Coulter), with a more pronounced effect on the latter, producing concentrations >20% lower than expected in the patient serum and spiked plasma pools. The effect was progressively negative at effective IgM concentrations of 10 and 15 mg/L. CONCLUSIONS: This phenomenon is a major analytical and clinical issue that must be communicated to health care professionals caring for patients receiving VANC, so optimal therapy is achieved.

The EMIT 2000 tacrolimus assay: an application protocol for the Beckman Synchron LX20 PRO analyzer.

OBJECTIVE: To develop and evaluate the performance of an application protocol for the EMIT 2000 tacrolimus (Tac) assay on the Beckman Synchron LX20 PRO Analyzer. DESIGN AND METHODS: Precision, accuracy, linearity, and lower limit of quantitation were investigated. Specimens from 212 kidney, liver, heart/heart-lung, and islet cell transplant patients were analyzed and results were compared to those from the Abbott MEIA II assay. A separate population of 232 specimens was coanalyzed by the enzyme-multiplied immunoassay technique (EMIT) assay and liquid chromatography tandem mass spectrometry (LC-MS/MS). RESULTS: Total imprecision was 13.7% and 6.0% at concentrations of 3.4 and 19.1 microg/L, respectively. Recoveries from assayed reference materials ranged from 103% to 109%. A quantitation range of 3.2-30.0 microg/L was validated. The EMIT assay on the LX20 PRO analyzer showed an average negative bias of 1% compared to the MEIA assay and an average positive bias of 17% compared to LC-MS/MS. CONCLUSION: This application for the EMIT 2000 Tac assay on the Beckman Synchron LX20 PRO analyzer enhances the versatility of the immunoassay for routine therapeutic drug monitoring (TDM) of this immunosuppressant in the clinical setting.