New insights on the analytical performances for detecting and quantifying monoclonal proteins

High-resolution capillary electrophoresis for the determination of carbamylated albumin

OBJECTIVES

Carbamylation is a non-enzymatic post-translational reaction of a primary amino group of a protein with isocyanate. The albumin carbamylation is a negative prognostic factor in chronic kidney disease (CKD) patients and induce charge difference implying an observed shift in electrophoretic mobility that can be measured through a symmetry factor (SF).

METHODS

The Helena V8 and the Sebia Capillarys 2 systems were used for all experiments. The effect of in vitro carbamylation on the SF by spiking increasing concentrations of potassium isocyanate (KCNO) in serum of three healthy volunteers was investigated. Theoretical plate numbers (N) as a surrogate of separation efficiency were also calculated and correlations between SF and renal function biomarkers were performed on 284 patients.

RESULTS

A dose-dependent impact of KCNO on the SF was observed for both methods with the Helena V8 being more sensitive. The mean N was significantly higher on the Helena V8 as compared to the Sebia Capillarys 2 (2,972 vs. 444.1, p<0.0001). The SF correlated significantly with eGFR (r=0.50, p<0.0001), creatinine (r=-0.31, p<0.0001) and urea (r=-0.34, p<0.0001) on the Helena V8. On the Sebia Capillarys 2, a significant correlation was only observed with eGFR (r=0.17, p=0.004). A better discrimination between CKD stages was also observed using the Helena V8.

CONCLUSIONS

Thanks to a higher mean N, the Helena V8 might offer new possibilities, including detection of carbamylated albumin through SF calculation. Further studies are still needed to confirm the interest of using this type of assays in clinical routine.

Evaluation of a Capillary Electrophoresis System for the Separation of Proteins

BACKGROUND

Serum protein electrophoresis is one of the core investigations for screening for monoclonal proteins. Among the available capillary systems, the Helena V8 system has been evaluated in a limited number of studies.

METHODS

In total, 310 sera samples were assessed on the Helena V8 system and compared with the Sebia Capillarys instrument. Abnormalities suggestive of monoclonal proteins were confirmed by immunofixation. Imprecision studies and reference intervals were determined.

RESULTS

The imprecision of the Helena V8 was inferior or equal to 5.8%. The mean bias of Helena V8 vs Sebia Capillarys was about -0.9 g/L for albumin; -0.2 g/L foralpha-1; 1.1 g/L for alpha-2; -0.2 g/L for beta; 0.3 g/L for gamma; -0.5 g/L for monoclonal protein in beta; and 0.3 g/L for monoclonal protein in gamma. Among the 56 samples with monoclonal proteins confirmed by immunofixation, all were seen on both methods, with only 1 discordant result at a cutoff of 5.0 g/L. Reference intervals were statistically different between the 2 analyzers, except for the beta fraction.

CONCLUSIONS

Our evaluation confirms the good analytical performance of the Helena V8 analyzer as a suitable alternative to the Sebia Capillarys instrument.

External quality assessment of M-protein diagnostics: a realistic impression of the accuracy and precision of M-protein quantification

OBJECTIVES

Studies that investigate the accuracy and precision of M-protein quantification are scarce. These studies are prone to give a biased view, since they are exclusively performed by institutions with international top-expertise on M-protein diagnostics. To obtain a realistic impression of the accuracy and precision of M-protein quantification, we studied results of 73 laboratories participating in the Dutch External Quality Assessment (EQA) program for M-protein diagnostics.

METHODS

To measure accuracy, healthy serum was spiked with respectively 1 and 5 g/L human IgG-kappa monoclonal antibody daratumumab. To measure precision, five sera were selected to be repeatedly send to all blinded EQA-participants.

RESULTS

The reported concentrations for the EQA-sample spiked with 5 g/L daratumumab ranged from 2.6 to 8.0 g/L (mean 4.9 g/L, between-laboratory CV = 23%). 98% of the participants detected and correctly characterized the 1 g/L daratumumab band. Both the accuracy (mean 1.7 g/L) and precision (between-laboratory CV = 46%) of this 1 g/L M-protein was poor. In the five EQA-samples that were repeatedly send to the same 73 participating laboratories, between-laboratory precision (mean CV = 25%) was significantly different than the within-laboratory precision (mean CV = 12%). Relatively poor precision was observed in sera with small M-proteins.

CONCLUSIONS

The EQA-data reveal a large variation in reported M-protein concentrations between different laboratories. In contrast, a satisfactory within-laboratory precision was observed when the same sample was repeatedly analyzed. The M-protein concentration is correlated with both accuracy and precision. These data indicate that M-protein quantification to monitor patients is appropriate, when subsequent testing is performed within the same laboratory.