B-336 Analytical verification of the EXENT® solution for the identification an quantitation of monoclonal immunoglobulins by mass spectrometry
Authors: Agulló C , Puig N , Contreras T , Castro S , Barbosa N , Diaz A , Pérez J , San Miguel J F , Lahuerta J , Bladé J , Mateos M V 
Published: October 2024

Abstract

Background
Recently a new mass spectrometry (MS) technology, EXENT® solution (The Binding Site, part of Thermo Fisher Scientific), for the identification and quantification of IgG, IgA and IgM intact monoclonal immunoglobulins (M-Ig) has been launched in the European markets. In this study we present the results of onsite verification study performed in our hospital.

Methods
Study design, using patients serum samples, including data analysis were based on the Clinical & Laboratory Standards Institute (CLSI). Relevant standards used were: Precision-CLSI EP15-A3:2014, Linearity-EP06 ED2:2020 and Comparison - EP09C ED3:2018. Samples were automatically prepared on the EXENT-iP®500 liquid handler, sample spots were analysed using the EXENT-iX®500 MALDI-TOF mass spectrometer. Light chain spectral peaks were identified by EXENT-iQ® software and then quantified indirectly in combination with total immunoglobulins concentrations by immunoturbidimetry in the Optilite® platform (The Binding Site, part of Thermo Fisher Scientific). We compared the results of MS with capillary electrophoresis (CZE) (IgG: n=82; IgA n=19; IgM n=19) and immunofixation (sIFE) (n=541).

Results
Precision was, for IgG with concentrations between 50.77 g/L and 3.51 g/L of 4.14% and 4.47%, for IgA between 61.88g/L and 5.02 g/L of 1.42% and 7.17%, and for IgM between 68.32 g/L and 7.30 g/L of 5.32% and 8.44%, respectively. Deviation from linearity of this assay, using serially diluted serum samples over the following ranges for IgG (65.18 g/L - 0.23 g/L), IgA (48.91 g/L - 0.26 g/L) and IgM (49.65 g/L -0.23 g/L) was &lt;10% for all isotypes. M-Ig concentration comparison for specimens with detectable IgG, IgA or IgM M-Ig with CZE, assessed by Passing Bablok regression was IgG g/L MS = -0.9756 + 1.523x CZE g/L (slope -2.058 to -0.3322 and intercept 1.331 to 1.679), IgA g/L MS = -2.114 + 1.646x CZE g/L (slope -12.50 to -0.5059 and intercept 1.105 to 3.296), IgM g/L MS = -1.886 + 1.935x CZE g/L (slope -5.205 to -0.3571 and intercept 1.239 to 2.462). All samples MS g/L = -1.182 + 1.556x CZE g/L (n=120). Agreement between EXENT/IFE was of 99.1% for IgGK(209/211), 97.2% IgGL(105/108), 100% AK(97/97), 100% AL(51/51), 100% MK(6/6), 100% ML(3/3), 89.2% free K(33/37), 89.3% free L(25/28), 66,7% non-secretory(4/6). Discordances intact immunoglobulin was due to samples with two M-Ig, where IFE wasn’t able to identify the major clone, for free K, free L was due to EXENT’s ability to identify the heavy chain that matched with the light chain, and for non-secretory patients due to EXENT was able to identify one M-Ig in 2/6 samples.

Conclusions
These on-site verification results demonstrate that the EXENT solution has a wide measuring interval, to detect M proteins with very low concentrations and to provide stable and reproducible performance with a high level of accuracy for the detection and typing of monoclonal immunoglobulins. However, the bias observed against CZE to quantify M-Ig would be justified for the lowest CZE sensitivity, CZE variability and also due to the CZE quantification uses the total protein concentration to quantify M-Ig.