Is accuracy of serum free light chain measurement achievable?

A Comparative Analysis of Freelite and N Latex FLC Assay for the sFLC Measurements in Suspected Patients of Monoclonal Gammopathy

With the introduction of newer platforms for sFLC estimation, more comparative attempts are required to establish an assay-specific reference range, that can be used in the day-to-day clinical management of the patients, as the revised criteria for the diagnosis of myeloma recommend the use of involved Free light chain (FLC)/uninvolved FLC ratio as myeloma defining event, solely relying on Freelite assay. This was an observational study that included 101 patients, 55 (54.5%) with M-protein (myeloma), and 46 (45.5%) without M-protein. The estimation of the sFLC assay was carried out at two separate platforms, the Freelite Assay (The binding Site, Birmingham, UK) on a Siemens BNII Nephelometer, and the N Latex FLC reagent on Atellica NEPH 630 (Siemens Healthineers, Eschborn, Germany). The comparative correlational analysis of the two assays for estimation of κ-, λ FLC, and κ/λ ratio showed a strong to a very strong association (P-value < 0.005) among total patients, however, this was reduced to weak to moderate association among patients with M-protein (P-value < 0.005). The contingency analyses to evaluate the agreement between the assays also showed a similar trend. A better concordance for the κ/λ ratio between the two assays was achieved while expanding this ratio. Our study concluded that two assays showed a better agreement among the patients without M-protein, but these two cannot be used interchangeably for the clinical management of myeloma patients.

Screening for and diagnosis of monoclonal gammopathy

Monoclonal gammopathy is a spectrum of disorders characterised by clonal proliferation of plasma cells or lymphocytes, which produce abnormal immunoglobulin or its components (monoclonal proteins). Monoclonal gammopathies are often categorised as low-tumour-burden diseases (eg, amyloid light chain (AL) amyloidosis), premalignant disorders (such as monoclonal gammopathy of undetermined significance and smouldering multiple myeloma), and malignancies (eg, multiple myeloma and Waldenström's macroglobulinaemia). Such diversity of concentration and structure makes monoclonal protein a challenging clonal marker. This article provides an overview on initial laboratory testing of monoclonal gammopathy to guide clinicians and laboratory professionals in the selection and interpretation of appropriate investigations.

Rapid detection of serum free light chains by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Introduction: Serum free light chain (FLC) measurements are increasingly prominent for patients with plasma cell disorders (PCDs) in screening, prognostic stratification, and monitoring therapy responses. Objectives: We aimed to develop a sensitive, reliable, and accurate method for diagnosing PCDs that can notably decrease the time and cost of current methods. Methods: Here, we present a novel approach for FLC measurement using immunoenrichment on micro-affinity chromatography in combination with matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) detection. In this study, serum free kappa (κ) and free lambda (λ) light chain (LC) levels in the serum of 105 patients were compared between the nephelometric serum FLC quantification and MALDI-TOF MS detection. Results: Cohen's kappa coefficient between the MALDI-TOF MS-based method and the FLC assay revealed an almost perfect agreement in the case of normal (negative) results (κ = 0.92; 95% confidence interval (CI): 0.837 to 0.968) and a good agreement in the case of increased (positive) results (κ = 0.76; 95% CI: 0.608 to 0.870). In Spearman's correlation analysis, the best correlation was found between serum free κ/λ ratios (r = 0.628, 0.496 to 0.732; p <0.0001). Our method showed sensitivity (92.5%) and specificity (76.3%) for discrimination between the κ/λ FLC ratio compared to the serum FLC assay. Conclusion: The proposed method can significantly contribute to diagnosing and monitoring PCDs as it can significantly be time-saving, cost-effective in FLC measurement.

Accuracy of determination of free light chains (Kappa and Lambda) in plasma and serum by Swedish laboratories as monitored by external quality assessment

BACKGROUND

Free light chain (FLC) measurements are important in diagnosing monoclonal gammopathies. As FLC are heterogeneous, different reagents and instruments for measuring FLC concentrations may give diverging results that affect assessment of patients with monoclonal gammopathies. Here we investigated agreement between different FLC methods using data from the Swedish external quality assurance (EQA) programme.

METHODS

The two main FLC assays, N Latex FLC (Siemens) and Serum Freelite (The Binding Site), using four nephelometric or turbidimetric instrument platforms, were compared. Results from 27 EQA rounds distributed to 11-16 Swedish hospital laboratories during 2015-2020 were investigated.

RESULTS

The kappa (κ) FLC measurements deviated significantly over time, but when only nephelometry was used, deviation from the mean was lower (median ranges: -5% to 13 %). The CV was significantly higher for the Freelite assay (mean CV = 8.7) than for the N latex assay (mean CV = 5.7) (p < 0.0001). The coefficient of determination between all combinations of reagents and instrument platforms used was generally good (r² = 0.76-0.87), and the correlation slope acceptable (0.81-1.2). For lambda (λ) FLC measurements, no concordance between combinations of instruments and reagents is apparent, deviating between -40 % to + 48 % from the mean. The CV was significantly higher for the combination with nephelometry and the Freelite assay (CV mean = 13.9 %) than nephelometry and the N latex assay (CV mean = 9.9 %) (p <0.001). The coefficient of determination varied between combinations of reagents and instrument platforms (r² = 0.59-0.89) and the slope ranged between 0.48 and 1.5. Significant differences between the two reagents used were sometimes noted.

CONCLUSIONS

Imprecision in λFLC affects the κFLC/λFLC ratio. This may be important in clinical assessment of patients, especially differentiating between monoclonal and polyclonal gammopathies.

Serum Free Light Chain Quantification Testing: Comparison of Two Methods for Disease Monitoring

Background

Levels of free immunoglobulin light chains in serum and urine are a sensitive measure of dysregulated immunoglobulin synthesis. The development of an assay for free light chains in serum was a major advance in laboratory testing for monoclonal gammopathies. The original assay by The Binding Site, called Freelite®, has been in common use in laboratory monitoring of monoclonal gammopathies. Two clinical entities, myeloma-defining condition and light chain-predominant multiple myeloma, rely on quantitative measurements of serum free light chains.

Methods

Using polyclonal antisera specific to free light chains, Diazyme Laboratories developed a latex immunoturbidimetric assay for quantification of human kappa and lambda serum free light chains. We evaluated the Diazyme assay by comparing the results of kappa and lambda free light chain quantification, and kappa/lambda ratio with the results on the same specimens by the Freelite method. We also compared the correlation of the 2 methods to evaluate response to treatment and to changes in clinical status of patients with multiple myeloma.

Results

The results of Freelite and Diazyme methods are comparable. There was no statistically significant difference in the performance of the 2 assays for quantification of light chains, kappa/lambda ratio, or correlation of clinical parameters from patients with multiple myeloma at various stages of monitoring the disease in 2 geographically diverse laboratory and clinical environments.

Conclusions

The Diazyme method is comparable to Freelite and provides an opportunity to add the test to front-end automation and improvement in efficiency of the assay.

Serum free light chain analysis: persisting limitations with new kids on the block

OBJECTIVES

Serum free light chain (sFLC) measurements have inherent analytical limitations impacting sFLC clinical interpretation. We evaluated analytical and diagnostic performance of three polyclonal sFLC assays on four analytical platforms.

METHODS

sFLC concentration was measured using Diazyme FLC assays (Diazyme) on cobas c501/c503 analyzer (Roche); Freelite assays (The Binding Site) on Optilite analyzer (The Binding Site) and cobas c501 analyzer and Sebia FLC ELISA assays (Sebia) on AP22 ELITE analyzer (DAS). Imprecision, linearity, method comparison vs. Freelite/Optilite, antigen excess detection and reference value verification were assessed. Diagnostic performance was compared on 120 serum samples and on follow-up samples of five patients with κ and λ monoclonal gammopathy.

RESULTS

Method comparison showed excellent correlation with Freelite/Optilite method for all assays. A large proportional negative bias was shown for both Sebia κ and λ ELISA and a significant positive proportional bias for λ in the low (<10 mg/L) Freelite/cobas c501 method. Clinically relevant underestimation of κ sFLC levels due to antigen excess was shown for 7% of each Diazyme/cobas application and for 11 and 32.1% of λ sFLC assay of respectively Diazyme/cobas and Sebia/AP22. sFLC reference values revealed application specific. Cohen's κ values were (very) good for κ sFLC but only moderate to good for λ sFLC. In 4/10 follow-up patients, significant differences in clinical interpretation between sFLC assays were noticed.

CONCLUSIONS

Important analytical limitations remain for all sFLC applications. Differences in reference values and diagnostic performance hamper interchangeability of sFLC assays. Assay specific sFLC decision guidelines are warranted.

Inter-Laboratory Concordance of Cerebrospinal Fluid and Serum Kappa Free Light Chain Measurements

The kappa index (K-Index), calculated by dividing the cerebrospinal fluid (CSF)/serum kappa free light chain (KFLC) ratio by the CSF/serum albumin ratio, is gaining increasing interest as a marker of intrathecal immunoglobulin synthesis. However, data on inter-laboratory agreement of these measures is lacking. The aim was to assess the concordance of CSF and serum KFLC measurements, and of K-index values, across different laboratories. KFLC and albumin of 15 paired CSF and serum samples were analyzed by eight participating laboratories. Four centers used Binding Site instruments and assays (B), three used Siemens instruments and assays (S), and one center used a Siemens instrument with a Binding Site assay (mixed). Absolute individual agreement was calculated using a two-way mixed effects intraclass correlation coefficient (ICC). Cohen's kappa coefficient (k) was used to measure agreement on positive (≥5.8) K-index values. There was an excellent agreement in CSF KFLC measurements across all laboratories (ICC (95% confidence interval): 0.93 (0.87-0.97)) and of serum KFLC across B and S laboratories (ICC: 0.91 (0.73-0.97)), while ICC decreased (to 0.81 (0.53-0.93)) when including the mixed laboratory in the analysis. Concordance for a positive K-Index was substantial across all laboratories (k = 0.77) and within S laboratories (k = 0.71), and very good (k = 0.89) within B laboratories, meaning that patients rarely get discordant results on K-index positivity notwithstanding the testing in different laboratories and the use of different platforms/assays.

Laboratory Detection and Initial Diagnosis of Monoclonal Gammopathies

CONTEXT.—

The process for identifying patients with monoclonal gammopathies is complex. Initial detection of a monoclonal immunoglobulin protein (M protein) in the serum or urine often requires compilation of analytical data from several areas of the laboratory. The detection of M proteins depends on adequacy of the sample provided, available clinical information, and the laboratory tests used.

OBJECTIVE.—

To develop an evidence-based guideline for the initial laboratory detection of M proteins.

DESIGN.—

To develop evidence-based recommendations, the College of American Pathologists convened a panel of experts in the diagnosis and treatment of monoclonal gammopathies and the laboratory procedures used for the initial detection of M proteins. The panel conducted a systematic literature review to address key questions. Using the Grading of Recommendations Assessment, Development, and Evaluation approach, recommendations were created based on the available evidence, strength of that evidence, and key judgements as defined in the Grading of Recommendations Assessment, Development, and Evaluation Evidence to Decision framework.

RESULTS.—

Nine guideline statements were established to optimize sample selection and testing for the initial detection and quantitative measurement of M proteins used to diagnose monoclonal gammopathies.

CONCLUSIONS.—

This guideline was constructed to harmonize and strengthen the initial detection of an M protein in patients displaying symptoms or laboratory features of a monoclonal gammopathy. It endorses more comprehensive initial testing when there is suspicion of amyloid light chain amyloidosis or neuropathies, such as POEMS (polyneuropathy, organomegaly, endocrinopathy, M protein, and skin changes) syndrome, associated with an M protein.

Using Two Detection Methods to Observe the Changes and Significance of Free Light Chain in Serum and Urine in Patients with Renal Insufficiency

Background

Free light chains κ and λ (FLC κ, FLC λ) are of great significance in diagnostic and monitoring monoclonal gammopathy. Freelite and N-Latex methods are two common monitoring methods at present. But the two meanings are not completely equivalent, especially for patients with renal insufficiency. We analyzed the changes of serum and urine FLC in renal insufficiency patients without monoclonal gammopathy and the clinical significance of these changes.

Methods

This study is an observational study. Patients ≥ 18 years old, who met the diagnostic criteria of chronic kidney disease (CKD), excluding monoclonal gammopathy, were selected. Fasting serum and 24-hour urine were taken to detect serum FLC κ, serum FLC λ, SCr, serum β₂-microglobulin, urinary FLC κ, urinary FLC λ, urinary α₁-microglobulin, and urinary β₂-microglobulin.

Results

There was a good correlation between the two methods for determining serum/urinary FLC. No matter serum or urine, FLC showed a good correlation with renal function by the N-Latex method, but not by the Freelite method. Under the N-Latex method, FLC κ/λ remained stable, which was basically within the reference range of healthy people and was not affected by renal function. There was a good correlation between FLC detected by N-Latex and microglobulin in serum and urine.

Conclusion

When the concentration of FLC is low, the N-Latex method is more recommended to monitor FLC. The FLC measured by the N-Latex method is more closely related to renal function. The ratio of FLC κ/λ determined by the N-Latex method remained stable within the recommended range.

Method comparison of three serum free light chain assays on the Roche Cobas 6000 c501 chemistry analyzer

OBJECTIVES

Free light chains (FLC) are important in the diagnosis, prognosis and monitoring of therapy response of patients with monoclonal gammopathies. In this study, we performed a method comparison of three FLC assays on the Cobas 6000 c501 chemistry analyzer of Roche Diagnostics.

METHODS

Samples of 119 patients with various monoclonal gammopathies and 26 control patients were measured with the Freelite (The Binding Site), Diazyme (Diazyme Laboratories) and KLoneus (Trimero Diagnostics) FLC assays. A method comparison was performed and reference intervals of the three assays were validated.

RESULTS

The analysis of the Bland-Altman agreement showed bias between the three FLC assays, ranging from -62.7 to 5.1% for κFLC and between -29.2 to 80.5% for λFLC. The Freelite and Diazyme assays have the highest agreement. The concordance of the FLC-ratio ranges from 41 to 75%, with the highest concordance between the Freelite and KLoneus assays. The FLC-ratio in 25 sera from healthy controls were within the reference ranges of the Freelite and KLoneus assays. The FLC-ratio was elevated in all 25 samples tested with the Diazyme assay.

CONCLUSIONS

The agreement for the free light chains is highest between the Freelite and the Diazyme assay and fair for the KLoneus assay. However, concordance of the FLC-ratio is highest when the Freelite and KLoneus assays were compared. Our data suggest that concordance for the Diazyme assay could be improved by recalibration. Because of absolute differences between the three methods in individual patients, none of the three FLC assays can be used interchangeably.

Baseline and Changes in Serum B-Cell Maturation Antigen Levels Rapidly Indicate Changes in Clinical Status Among Patients with Relapsed/Refractory Multiple Myeloma Starting New Therapy

BACKGROUND

B-cell maturation antigen (BCMA) is expressed on malignant plasma cells from patients with multiple myeloma (MM). These patients have higher levels of serum (s)BCMA than healthy subjects, and levels correlate with disease status. The half-life of sBCMA is only 24-36 h, and levels are independent of renal function.

OBJECTIVE

We determined whether baseline sBCMA values, a ≥ 25% increase, and a ≥ 50% decrease during treatment predicted progression-free survival (PFS) and overall survival (OS) among 81 patients with relapsed/refractory MM (RRMM) starting new treatments.

METHODS

Serum was obtained on day 22 of each patient's 28-day cycle of new therapy. Kaplan-Meier survival analysis and log-rank comparison tests were used to determine the effect of baseline sBCMA. The effect of percentage change in sBCMA was investigated using time-dependent Cox proportional hazard models.

RESULTS

Patients with baseline sBCMA levels above the median had a shorter PFS (p = 0.0077), and those in the highest quartile had a shorter PFS (p = 0.0012) and OS (p = 0.0022). A ≥ 25% increase at week 4, week 8, and anytime through week 12 predicted a shorter PFS (p = 0.0011, p = 0.0005, and p < 0.0001, respectively). A ≥ 50% decrease at week 4, week 8, and anytime through week 12 predicted a longer PFS (p = 0.0045, p = 0.029, p = 0.0055, respectively). A ≥ 25% increase in sBCMA occurred before progression according to International Myeloma Working Group criteria in 67.5% of patients.

CONCLUSIONS

Our results indicate the potential for the use of sBCMA as a new biomarker for monitoring patients with RRMM.

FLC polymerization: Another hurdle towards standardization of FLC measurements

In the November issue of this journal, Caponi et al. convincingly show that free light chain (FLC) lambda measurements are influenced by the FLC-polymerization status. With liquid chromatography using a size-exclusion column they separated monoclonal FLC dimers from monomers. FLC analysis of these different fractions clearly indicated that Freelite (The Binding Site) reacted better with FLC dimers than with FLC monomers. In contrast, N Latex FLC (Siemens) recognizes mainly FLC monomers and has poor reactivity with the FLC dimers. This important finding may be one of the leading factors to explain discrepancies in FLC-quantification using Freelite versus other immunochemical assays such as N-Latex FLC.

Serum and Urine Protein Electrophoresis and Serum-Free Light Chain Assays in the Diagnosis and Monitoring of Monoclonal Gammopathies

BACKGROUND

Laboratory methods for diagnosis and monitoring of monoclonal gammopathies have evolved to include serum and urine protein electrophoresis, immunofixation electrophoresis, capillary zone electrophoresis, and immunosubtraction, serum-free light chain assay, mass spectrometry, and newly described QUIET.

CONTENT

This review presents a critical appraisal of the test methods and reporting practices for the findings generated by the tests for monoclonal gammopathies. Recommendations for desirable practices to optimize test selection and provide value-added reports are presented. The shortcomings of the serum-free light chain assay are highlighted, and new assays for measuring monoclonal serum free light chains are addressed.

SUMMARY

The various assays for screening, diagnosis, and monitoring of monoclonal gammopathies should be used in an algorithmic approach to avoid unnecessary testing. Reporting of the test results should be tailored to the clinical context of each individual patient to add value. Caution is urged in the interpretation of results of serum-free light chain assay, kappa/lambda ratio, and myeloma defining conditions. The distortions in serum-free light chain assay and development of oligoclonal bands in patients' status post hematopoietic stem cell transplants is emphasized and the need to note the location of original monoclonal Ig is stressed. The need for developing criteria that consider the differences in the biology of kappa and lambda light chain associated lesions is stressed. A new method of measuring monoclonal serum-free light chains is introduced. Reference is also made to a newly defined entity of light chain predominant intact immunoglobulin monoclonal gammopathy. The utility of urine testing in the diagnosis and monitoring of light chain only lesions is emphasized.

Mass Spectrometry for Identification, Monitoring, and Minimal Residual Disease Detection of M-Proteins

BACKGROUND

Monoclonal gammopathies (MGs) are plasma cell disorders defined by the clonal expansion of plasma cells, resulting in the characteristic excretion of a monoclonal immunoglobulin (M-protein). M-protein detection and quantification are integral parts of the diagnosis and monitoring of MGs. Novel treatment modalities impose new challenges on the traditional electrophoretic and immunochemical methods that are routinely used for M-protein diagnostics, such as interferences from therapeutic monoclonal antibodies and the need for increased analytical sensitivity to measure minimal residual disease.

CONTENT

Mass spectrometry (MS) is ideally suited to accurate mass measurements or targeted measurement of unique clonotypic peptide fragments. Based on these features, MS-based methods allow for the analytically sensitive measurement of the patient-specific M-protein.

SUMMARY

This review provides a comprehensive overview of the MS methods that have been developed recently to detect, characterize, and quantify M-proteins. The advantages and disadvantages of using these techniques in clinical practice and the impact they will have on the management of patients with MGs are discussed.

Evaluation of two serum free light chain quantitation methods, Freelite and Seralite, in the clinical laboratory with a view to switching immunoassay

BACKGROUND

Serum free light chain (sFLC) quantitation is central for plasma cell dyscrasias. Several assays are available and switching sFLC methods may be advantageous in certain laboratories. This study performed Freelite and Seralite simultaneously for samples received by the clinical laboratory over a 10 month period and compared quantitation and its impact on interpretation of patient results.

METHODS

Patients (N = 189) included multiple myeloma (MM) and related plasma cell cancers, monoclonal gammopathy of unknown significance (MGUS), AL amyloidosis and renal impairment. sFLC quantitation and clinical agreement was assessed between methods.

RESULTS

Clinical agreement was substantial at diagnosis (κ = 0.647, p < .01) and moderate for monitoring (κ = 0.591, p < .01). Good concordance was seen for MM and related plasma disorders and MGUS, with poorer agreement seen for AL amyloidosis. Case studies illustrated agreement in pattern of myeloma disease activity. Bland-Atman plots showed small mean bias but increasing variation between methods with increasing FLC concentrations. Passing-Bablok analysis confirmed systematic differences in quantitation between methods.

CONCLUSIONS

Despite differences in quantitation, overall, agreement was seen between the different sFLC platforms in relation to the clinical interpretation. As a rapid test without the need for large and expensive analysers, Seralite may be highly applicable in certain laboratories to enable in-house testing.

Comparison of three different serum-free light-chain assays-implications on diagnostic and therapeutic monitoring of multiple myeloma

The measurement of serum-free light chains (FLC) is standard of care in the diagnosis and management of multiple myeloma (MM). The revised international myeloma working group (IMWG) implemented the involved FLC/noninvolved FLC (iFLC/niFLC) ratio as a biomarker for MM requiring treatment. Recently, a new definition of high-risk smoldering MM (SMM) including iFLC/niFLC ratio was published. These recommendations were solely based on a single assay method (Freelite assay). Today, two additional assays, N Latex FLC and ELISA-based Sebia FLC, are available. Here, we report on a single-center-study comparing results of all three different assays for FLC correlation and its potential implications for diagnostic and clinical use. In total, 187 samples from 47 MM patients were examined, and determination of FLC was performed. Comparison analyses showed similar FLC results for Sebia FLC and N Latex FLC assay with markedly lower absolute values for κ/λ ratio compared with Freelite. Values of λ FLC exhibited high variability. The ratio of iFLC/niFLC showed significant discrepancies among these assays. Our data demonstrate that the three available assays may result in markedly discrepant results, and should not be used interchangeably to monitor patients. Furthermore, modifications of the assay-specific diagnostic (iFLC/niFLC) thresholds for SMM and MM are recommended.

Method comparison of four clinically available assays for serum free light chain analysis

Background

Serum free light chain (sFLC) measurements are increasingly important in the context of screening for monoclonal gammopathies, prognostic stratification and monitoring of therapy responses. In this study we have performed a method comparison of four sFLC assays that are currently available for routine clinical use.

Methods

In a retrospective study, sFLC analyses were performed on a cohort that included 139 patients with various monoclonal gammopathies and 54 control sera without an M-protein. Method comparisons of the following four FLC assays were performed: Freelite (Binding Site), N-Latex FLC (Siemens), Seralite (Abingdon Health) and Sebia FLC (Sebia).

Results

Bland-Altman agreement analysis showed biases varying between −0.1 and 16.2 mg/L for κFLC, −6.0 and 6.8 mg/L for λFLC and −0.04 and 0.38 for the ratio of the involved to uninvolved FLC. Strong agreements were observed for FLC-concentrations below 100 mg/L. The clinical concordance of the κ/λFLC-ratio of the four methods varied between 86% and 92%. Significant quantitative differences were observed between the different methods, mainly in sera with high FLC concentrations. Most assays consistently overestimated FLC concentrations compared to SPE.

Conclusions

Good overall clinical concordances were observed between the four sFLC assays that were compared in this study. Although good agreements were observed between the FLC assays, significant absolute differences in FLC concentrations in individual patients can be seen, particularly at higher FLC concentrations. Because of inequivalent absolute sFLC values between the methods in individual patients, none of the four sFLC assays can be used interchangeably.

Inter-assay variability in automated serum free light chain assays and their use in the clinical laboratory

Serum κ and λ free light chain levels are markers of plasma cell proliferation, and their measurements have been included in recent guidelines by the International Myeloma Working Group for the management of patients with plasma cellular dyscrasias. Five in vitro diagnostic methods for the immunochemical quantification of serum free light chains (FLC) are available, three based on polyclonal antibodies (Freelite^®, The Binding Site; FLC ELISA κ and λ, Sebia; human κ and λ FLC, Diazyme Laboratories) and two on monoclonal antibodies (N Latex FLC, Siemens Healthineers; Seralite^®, Sebia). Several studies have shown that these methods cannot be used interchangeably for the follow-up of patients because measured κ and λ FLC concentrations may differ significantly, especially at high levels. Because no international reference material for the measurement of FLC is available, it is not possible to establish which method is the most accurate. For this reason, knowledge about the analytical and diagnostic performances of the assays used is important. The aim of this review is to describe the main analytical features of the κ and λ FLC assays and how they may influence the clinical use of these parameters.

Risk stratification of smoldering multiple myeloma: predictive value of free light chains and group-based trajectory modeling

We investigated the predictive role for serum free light chain ratio (FLCr) ≥100, bone marrow plasma cell (BMPC) ≥60%, and evolving biomarkers through group-based trajectory modeling (GBTM) as high-risk defining events in 273 smoldering multiple myeloma (SMM) patients with a median follow-up of 74 months. FLCr ≥100 was confirmed as a marker for high-risk progression with a median time to progression (TTP) of 40 months with a 44% risk of progression of disease (PD) at 2 years; however, 44% of FLCr ≥100 also did not progress during follow-up. For patients with BMPC ≥60% by core biopsy, the median TTP was 31 months with a 2-year PD of 41%. GBTM established high-risk trajectories for evolving hemoglobin (eHb; characterized as a 1.57 g/dL decrease in hemoglobin), evolving m-protein (eMP; 64% increase in m-protein), and evolving differences in FLC (edFLC; 169% increase in dFLC) within 1 year of diagnosis associated with a decreased median TTP and an increased 2 year rate of PD. Of all the variables examined, we identify a model where immunoparesis, eHb, eMP, and edFLC were significant predictors for ultra-high-risk progression with a median TTP of only 13 months with 3 or more variables present. Our results not only confirm a more modest 2 year PD associated with FLCr ≥100 and BMPC ≥60 but also suggest that eHb, eMP, and edFLC may help identify an ultra-high-risk SMM group.

Free light chains: Eclectic multipurpose biomarker

The production of antibodies is accompanied by a slight excess of synthesis of κ and λ immunoglobulin light chains; small amounts of them are released in the peripheral blood and can also be found in various body fluids, such as synovial fluid, cerebrospinal fluid, urine and saliva. They are rapidly filtered by the glomerulus and >99% are reabsorbed from the cells of the proximal convoluted tubule, making them present in the urine in only trace amounts. The production of an excess of protein without a reason or a specific function in a biological system is rare. Free light chains, considered for years a waste product of Ig synthesis, are currently known to be very active molecules, able to bind antigens as well as whole immunoglobulin and helping to develop specific antibody affinity. The ability of free light chains to activate mast cells and then become an active part of the pathogenic mechanisms of chronic inflammatory diseases has increased interest in their clinical use, both as an attractive therapeutic target or as a biochemical marker of disease evolution or remission. This is an overview of relevant scientific interest that immunoglobulin light chains κ and λ have attracted over the years, a report on the progress in knowledge about their structure and function, with a special focus on their biological meaning and potential clinical utility in different diseases.

Evaluation of a new free light chain ELISA assay: bringing coherence with electrophoretic methods

Background:

Serum free light chain (sFLC) measurements are increasingly important in the context of screening for monoclonal gammopathies, prognostic stratification, and monitoring of therapy responses. At the same time, analytical limitations have been reported with the currently available nephelometric and turbidimetric sFLC assays. We have evaluated a new quantitative sFLC ELISA for its suitability in routine clinical use.

Methods:

Reference ranges of the Sebia FLC assay were calculated from 208 controls. Assay interference, reproducibility, lot-to-lot variability, and linearity were assessed. Method comparison to the Freelite assay (Binding Site) was conducted by retrospective analysis of 501 patient sera.

Results:

Reference ranges of the Sebia κ/λFLC-ratio were 0.37–1.44. We observed good sensitivity (1.5 mg/L) and linearity in both polyclonal and monoclonal sFLC samples and never experienced antigen excess. Sebia FLC reproducibility varied between 6.7% and 8.1% with good lot-to-lot consistency. Method comparison with Freelite showed the following correlations: κFLC R=0.94, λFLC R=0.92 and κ/λFLC-ratio R=0.96. The clinical concordance of the κ/λFLC-ratio of both methods was 94%. Significant quantitative differences were observed between both methods, mainly in sera with high FLC concentrations. The Sebia monoclonal FLC concentrations were coherent with those obtained by serum protein electrophoresis (SPE). Freelite monoclonal FLC concentrations were consistently higher, with a mean 12-fold overestimation compared to SPE.

Conclusions:

The Sebia FLC assay provides a novel platform for sensitive and accurate sFLC measurements. The Sebia FLC showed good clinical concordance with Freelite. Further studies are warranted to confirm the clinical value of this assay.

Serum free light chain quantitative assays: Dilemma of a biomarker

BACKGROUND

Serum free light chains detection assays are consistently meeting greater interest for the diagnosis and monitoring of monoclonal gammopathies and plasma cell dyscrasias. Nowadays, there are neither standardized methods nor reference material for the determination of free light chains; for this reason, it is important to compare two different assays used in clinical laboratory.

METHODS

We evaluated 300 serum samples from patients with B-cell disorders and compared the analytical performances of both assay. Each test was assayed on both testing platforms (Siemens Dade Behring BN II Nephelometer and SPAPLUS by The Binding Site). κ/λ ratios were determined and compared. Results were analyzed by Passing-Bablok and Bland-Altman plots to evaluate comparability of the two techniques and to determine bias.

RESULTS

The reproducibility of both assays is acceptable, reaching minimum and desirable analytical goals derived from biological variability. However, values are not interchangeable between systems. This study shows that the two systems do not allow results to be transferred from one method to the other even if they display good agreement.

CONCLUSION

Our study highlights the importance of elaborating an international standard for free light chains quantification in order to offer homogeneous results as well as guarantee harmonization of values among laboratories. Moreover, the assays should be validated in specific patient groups to determine that they are clinically fit for purpose.

Analytical Criticalities Associated to Different Immunological Methods for Serum Free Light Chain Detection in Plasma Cell Dyscrasias: A Description of Particular Clinical Cases

Current criteria for differential diagnosis of multiple myeloma (MM), Monoclonal gammopathy of undetermined significance (MGUS), and smoldering multiple myeloma (SMM) are included in the 2003 guidelines by the International Myeloma Working Group (IMWG). An updated version was then published in 2014, highlighting the importance of serum free light chain (sFLC) detection, as well as the κ/λ ratio as excellent indicators of clonality. At present, two commercial assays for sFLC quantification are available: the Freelite™ assay and the N-Latex assay. The first was developed by The Binding Site based on a mixture of polyclonal antibodies directed against a variety of FLC epitopes. It may be run on a wide range of nephelometers, as well as on turbidimeters. The second method was developed by Siemens and runs exclusively on Siemens instruments. It employs a probe mixture of mouse monoclonal antibodies. The aim of our study was to evaluate sFLC measurement and calculated κ/λ ratio in 85 patients with monoclonal gammopathies (MGs) in order to compare methods. We demonstrated that there is only a moderate concordance between the two FLC assays. In particular, in one case, we observed no qualitative alterations of the serum protein pattern, and in the absence of a Freelite™ assay, sFLC measurement would not have been possible to highlight the increase of λ FLC.

Serum B-cell maturation antigen: a novel biomarker to predict outcomes for multiple myeloma patients

B-cell maturation antigen is expressed on plasma cells. In this study, we have identified serum B-cell maturation antigen as a novel biomarker that can monitor and predict outcomes for multiple myeloma patients. Compared to healthy donors, patients with multiple myeloma showed elevated serum B-cell maturation antigen levels (P<0.0001). Serum B-cell maturation antigen levels correlated with the proportion of plasma cells in bone marrow biopsies (Spearman's rho = 0.710; P<0.001), clinical status (complete response vs partial response, P=0.0374; complete response vs progressive disease, P<0.0001), and tracked with changes in M-protein levels. Among patients with non-secretory disease, serum B-cell maturation antigen levels correlated with bone marrow plasma cell levels and findings from positron emission tomography scans. Kaplan-Meier analysis demonstrated that serum B-cell maturation antigen levels above the median levels were predictive of a shorter progression-free survival (P=0.0006) and overall survival (P=0.0108) among multiple myeloma patients (n=243). Specifically, patients with serum B-cell maturation antigen levels above the median level at the time of starting front-line (P=0.0043) or a new salvage therapy (P=0.0044) were found to have shorter progression-free survival. Importantly, serum B-cell maturation antigen levels did not show any dependence on renal function and maintained independent significance when tested against other known prognostic markers for multiple myeloma such as age, serum β2 microglobulin, hemoglobin, and bone disease. These data identify serum B-cell maturation antigen as a new biomarker to manage multiple myeloma patients.