LETTERS
Paraprotein Interference in Immunoassays To the Editor: We wish to supplement the findings of an article recently published in Therapeutic Drug Monitoring1 in which the authors draw attention to the deleterious effect of immunoglobulin M (IgM) paraproteins on the quantification of phenytoin using a particle-enhanced turbidimetric inhibition (PETINA) assay. The negative effect of IgM paraproteins on the quantification of vancomycin using PETINA assays was described by us in an article published in Therapeutic Drug Monitoring in 2012.2 In our article, we reported that paraproteins of IgG, IgM, and IgA did not affect quantitative measurements of vancomycin in fluorescence polarization or enzyme multiplied immunoassays but IgM paraproteins showed interference in PETINA assays. The interference was characterized as attenuation of vancomycin serum and plasma concentrations (.20% decrease) at IgM concentrations .10g/L. The phenomenon was progressive with more pronounced effect occurring at higher IgM concentrations. Individuals involved in clinical laboratory analysis need to be aware of the deleterious effect of high concentration of paraproteins on various laboratory measurements and need to investigate when laboratory findings are not in agreement with clinical findings. Donald F. LeGatt, PhD, FCACB Trefor N. Higgins, FCACB†
TO THE
EDITOR
with a high level of IgM. Ther Drug Monit. 2014;36:553–555. 2. LeGatt DF, Blakney GB, Higgins TN, et al. The effect of paraproteins and rheumatoid factor on four commercial immunoassays for vancomycin: implications for laboratorians and other health care professionals. Ther Drug Monit. 2012;34:306–311.
Unexpected Overestimation of Methotrexate Plasma Concentrations: Importance of the Assay Implementation and Its Validation
1. Hirata K, Saruwatari J, Enoki Y, et al. Possible false-negative results on therapeutic drug monitoring of phenytoin using a particle enhanced turbidimetric inhibition immunoassay in a patient
To the Editors: The article by Dr. Guerriero et al1 is a useful and welcome article concerning a worthy alternative for fluorescence polarization immunoassay (FPIA) for the quantitative determination of methotrexate (MTX) in human serum or plasma. Articles describing this topic are valuable, as FPIA is gradually disappearing, and alternative assays2,3 are needed. However, we believe that the article by Guerriero et al1 has some major shortcomings. This article did not cite some recent and relevant publications2,4,5 on the validation of the alternative MTX assays. Moreover, the article did not include sufficient information regarding the laboratory method used, making it impossible to compare these results with previously published studies. In contrast with the FPIA assay, the alternative MTX homogeneous enzyme immunoassays are frequently installed in open channel on a variety of random access chemistry analyzers. As applications might be customized when installed in open channel,2,4,5 it is important to include the exact application protocol used. Recently, a thorough multicenter method evaluation of the ARK MTX Immunoassay4 (ARK Diagnostics, Fremont, CA) was conducted
The authors declare no conflict of interest.
The authors declare no conflict of interest.
*Department of Laboratory Medicine and Pathology, University of Alberta Hospital, Edmonton, Canada †Department of Clinical Biochemistry, DynaLIFEDX, Edmonton, Canada
REFERENCES
Ther Drug Monit Volume 37, Number 3, June 2015
on 4 different automated clinical chemistry analyzers. This study showed that the analytical performance of the ARK assay differs between analyzers, and a drift was observed for the zero calibrator on some, but not all, analyzers. In the article by Dr. Guerriero et al,1 neither the analyzer nor the application protocol used for the ARK assay or Syva Emit MTX assay (Siemens, Tarrytown, NY) were mentioned. The latter assay (standard protocol) was in January 2012 also briefly evaluated on the c502 module of the Cobas 8000 (Roche Diagnostics, Mannheim, Germany) in our laboratory at Ghent University Hospital and was found to be impractical (reconstitution and equilibration steps for the preparation of the different components of the kit) and inadequate (high CV, bias, and unacceptable total error for low control material, unpublished data, M. Godefroid). To improve the sensitivity and reliability of the Syva Emit MTX assay, modifications have been suggested for this assay.2,5 As the standard application protocol of the Syva assay is referring to a MTX measuring range of 0.3–2 mmol/L and the laboratory in the article of Guerriero et al1 was still able to report MTX results #0.2 mmol/L, we suppose that the laboratory was also using an optimized application protocol. However, because of some major limitations in the initial evaluation of this assay in 2009, as described by Guerriero et al1 [method comparison based on internal quality control (IQC) material and MTXspiked serum samples only], the laboratory did not pick up the difference in accuracy between the FPIA reagent and the Syva assay. The authors claim that the aforementioned differences in accuracy and the overestimation in MTX measurements between 2009 and 2012 were not noticed with daily IQC and hence were unexpected. When considering the target values of the IQC materials that were used in this study, the IQC material from Randox Laboratories (used with the Syva assay) was not appropriate to evaluate and monitor MTX kit performance at the lower measuring end, as the lowest control level had a target value of 0.354 mmol/L,
417
Copyright © 2015 Wolters Kluwer Health, Inc. Unauthorized reproduction of this article is prohibited.
Paraprotein Interference in Immunoassays To the Editor: We wish to supplement the findings of an article recently published in Therapeutic Drug Monitoring1 in which the authors draw attention to the deleterious effect of immunoglobulin M (IgM) paraproteins on the quantification of phenytoin using a particle-enhanced turbidimetric inhibition (PETINA) assay. The negative effect of IgM paraproteins on the quantification of vancomycin using PETINA assays was described by us in an article published in Therapeutic Drug Monitoring in 2012.2 In our article, we reported that paraproteins of IgG, IgM, and IgA did not affect quantitative measurements of vancomycin in fluorescence polarization or enzyme multiplied immunoassays but IgM paraproteins showed interference in PETINA assays. The interference was characterized as attenuation of vancomycin serum and plasma concentrations (.20% decrease) at IgM concentrations .10g/L. The phenomenon was progressive with more pronounced effect occurring at higher IgM concentrations. Individuals involved in clinical laboratory analysis need to be aware of the deleterious effect of high concentration of paraproteins on various laboratory measurements and need to investigate when laboratory findings are not in agreement with clinical findings. Donald F. LeGatt, PhD, FCACB Trefor N. Higgins, FCACB†
TO THE
EDITOR
with a high level of IgM. Ther Drug Monit. 2014;36:553–555. 2. LeGatt DF, Blakney GB, Higgins TN, et al. The effect of paraproteins and rheumatoid factor on four commercial immunoassays for vancomycin: implications for laboratorians and other health care professionals. Ther Drug Monit. 2012;34:306–311.
Unexpected Overestimation of Methotrexate Plasma Concentrations: Importance of the Assay Implementation and Its Validation
1. Hirata K, Saruwatari J, Enoki Y, et al. Possible false-negative results on therapeutic drug monitoring of phenytoin using a particle enhanced turbidimetric inhibition immunoassay in a patient
To the Editors: The article by Dr. Guerriero et al1 is a useful and welcome article concerning a worthy alternative for fluorescence polarization immunoassay (FPIA) for the quantitative determination of methotrexate (MTX) in human serum or plasma. Articles describing this topic are valuable, as FPIA is gradually disappearing, and alternative assays2,3 are needed. However, we believe that the article by Guerriero et al1 has some major shortcomings. This article did not cite some recent and relevant publications2,4,5 on the validation of the alternative MTX assays. Moreover, the article did not include sufficient information regarding the laboratory method used, making it impossible to compare these results with previously published studies. In contrast with the FPIA assay, the alternative MTX homogeneous enzyme immunoassays are frequently installed in open channel on a variety of random access chemistry analyzers. As applications might be customized when installed in open channel,2,4,5 it is important to include the exact application protocol used. Recently, a thorough multicenter method evaluation of the ARK MTX Immunoassay4 (ARK Diagnostics, Fremont, CA) was conducted
The authors declare no conflict of interest.
The authors declare no conflict of interest.
*Department of Laboratory Medicine and Pathology, University of Alberta Hospital, Edmonton, Canada †Department of Clinical Biochemistry, DynaLIFEDX, Edmonton, Canada
REFERENCES
Ther Drug Monit Volume 37, Number 3, June 2015
on 4 different automated clinical chemistry analyzers. This study showed that the analytical performance of the ARK assay differs between analyzers, and a drift was observed for the zero calibrator on some, but not all, analyzers. In the article by Dr. Guerriero et al,1 neither the analyzer nor the application protocol used for the ARK assay or Syva Emit MTX assay (Siemens, Tarrytown, NY) were mentioned. The latter assay (standard protocol) was in January 2012 also briefly evaluated on the c502 module of the Cobas 8000 (Roche Diagnostics, Mannheim, Germany) in our laboratory at Ghent University Hospital and was found to be impractical (reconstitution and equilibration steps for the preparation of the different components of the kit) and inadequate (high CV, bias, and unacceptable total error for low control material, unpublished data, M. Godefroid). To improve the sensitivity and reliability of the Syva Emit MTX assay, modifications have been suggested for this assay.2,5 As the standard application protocol of the Syva assay is referring to a MTX measuring range of 0.3–2 mmol/L and the laboratory in the article of Guerriero et al1 was still able to report MTX results #0.2 mmol/L, we suppose that the laboratory was also using an optimized application protocol. However, because of some major limitations in the initial evaluation of this assay in 2009, as described by Guerriero et al1 [method comparison based on internal quality control (IQC) material and MTXspiked serum samples only], the laboratory did not pick up the difference in accuracy between the FPIA reagent and the Syva assay. The authors claim that the aforementioned differences in accuracy and the overestimation in MTX measurements between 2009 and 2012 were not noticed with daily IQC and hence were unexpected. When considering the target values of the IQC materials that were used in this study, the IQC material from Randox Laboratories (used with the Syva assay) was not appropriate to evaluate and monitor MTX kit performance at the lower measuring end, as the lowest control level had a target value of 0.354 mmol/L,
417
Copyright © 2015 Wolters Kluwer Health, Inc. Unauthorized reproduction of this article is prohibited.
