Abstract

Measurement of free kappa and free lambda light chains plays a key role in diagnosis, monitoring, and prognosis for many patients with a monoclonal gammopathy. These assays are advocated by the International Myeloma Working Group (IMWG) as an essential component of the primary screening algorithm for suspected monoclonal plasma cell disorders.18 Markedly increased baseline clonal free light chain concentrations are associated with poor prognosis in amyloid light chain (AL) amyloidosis, multiple myeloma, and virtually every plasma cell disorder.18 An abnormal serum free light chain ratio at baseline is predictive of an increased risk of progression in patients with monoclonal gammopathy of undetermined significance.20 Serial monitoring with serum free light chain analysis is recommended by the IMWG for patients with oligosecretory myeloma and AL amyloidosis. Additional data is needed to critically assess the utility of serum free light chain analysis as a serial monitoring technique in patients with multiple myeloma. The significance of isolated serum free light chain ratio abnormalities (“free light chain monoclonal gammopathy of undetermined significance”) also needs to be explored.

The serum free light chain assays (Freelite, The Binding Site Ltd., Birmingham, U.K.) for free kappa and free lambda light chains were first commercially available in 2001. These nephelometric/turbidimetric assays measure free kappa and lambda light chains using latex-conjugated polyclonal antibodies to epitopes that are sequestered when light chains are bound to heavy chains, but exposed when light chains circulate freely. This highly sensitive technique enables fully automated measurement of circulating free light chains. The first clinical utility of serum free light chain analysis was in the detection of measurable levels of circulating clonal free light chains in “nonsecretory” myeloma—now reclassified as oligosecretory myeloma—thus enabling these patients to be monitored in the course of treatment for the first time. This not only allowed for better management of these patients, but also allowed them to be enrolled in clinical trials.1,2 Since then, additional applications have been described. Serum free light chain analysis is now recommended as an integral part of a serum panel for the initial diagnosis of monoclonal gammopathy, for serial monitoring of patients with oligosecretory disease, and for prognosis in virtually every plasma cell dyscrasia as described below.

Interpretation

Reference intervals for the serum free light chain assays (Table 1) have been established in normal subjects and proposed for those with chronic kidney disease.3,4 In patients with polyclonal hypergammaglobulinemia or renal impairment, free kappa and free lambda concentrations may increase due to increased immune production or decreased clearance, respectively.3,5 Use of newly proposed “renal” reference intervals for the kappa/lambda ratio improved the specificity of these assays from 93% to 99% in a study of 142 patients with dialysis-dependent renal failure.6

Free light chain testing is considered abnormal when the clonal free light chain is increased above normal and the ratio is abnormal. Measurable disease with free light chain testing has been defined as a clonal free light chain concentration of at least 100 mg/L with an abnormal ratio.7 Approaches to reporting serum free light chain testing have used kappa-lambda plots and, for patients being serially monitored, percent changes from prior results.8

Table 1

Reference Intervals for Serum Free Light Chains3,4

Analyte Reference Interval 
Free kappa 3.3–19.4 mg/L 
Free lambda 5.7–26.3 mg/L 
Kappa/lambda ratio without renal impairment 0.26–1.65 
Kappa/lambda ratio with renal impairment 0.37–3.1 
Analyte Reference Interval 
Free kappa 3.3–19.4 mg/L 
Free lambda 5.7–26.3 mg/L 
Kappa/lambda ratio without renal impairment 0.26–1.65 
Kappa/lambda ratio with renal impairment 0.37–3.1 

Diagnosis

A serum panel using serum free light chain analysis in combination with either serum protein electrophoresis or serum immunofixation has been shown in several studies to be a highly sensitive approach to screening for most plasma cell disorders.9–12 The sensitivity of serum free light chain testing approaches 100% for the diagnosis of light chain myeloma,13 is more than 80% in light chain deposition disease,14 and is generally greater than 90% in amyloid light chain (AL) amyloidosis where these assays often detect disease despite negative serum and/or urine immunofixation.4,11,15,16 In rare instances, urine immunofixation detects AL amyloidosis in patients despite normal serum free light chain testing.15 Therefore, urine and serum studies are complementary tests when evaluating patients with suspected AL amyloidosis.15 In patients with intact immunoglobulin myeloma, up to 96% have been shown to have concomitant abnormalities with serum free light chain testing.17

Based on the accumulated evidence, the International Myeloma Working Group (IMWG) has recommended a serum panel consisting of serum free light chain analysis in combination with serum protein electrophoresis and serum immunofixation when evaluating for monoclonal plasma cell disorders for all patients except AL amyloidosis for which a 24-h urine immunofixation is still also required.18 A major limitation of urine immunofixation is that it is not quantitative. Urine free light chain testing may have value in this setting, but its potential benefit has not been evaluated to date.

Serum free light chain concentrations may be markedly increased at presentation. Values as high as 100,000 mg/L have been reported. Samples with very high free light chain levels require serial dilution to obtain a quantitative result. Rarely, a spuriously low baseline serum free light chain result may be reported if antigen excess is not detected by automated nephelometry or turbidimetry (the hook effect).19 Strategies to prevent reporting of spuriously low values due to antigen excess include diluting specimens with an unusually large change in concentration from a prior specimen (delta check) and diluting all specimens with detectable monoclonal proteins by electrophoresis or immunofixation but normal free light chain results.19 The latter situation is often observed with monoclonal gammopathy of undetermined significance (MGUS) in which two-thirds of patients have a normal serum free light chain ratio,20 but may also occur with antigen excess.

Monitoring Response to Treatment

Serum free light chain analysis is a valuable tool when monitoring response to chemotherapy in patients with light chain-secreting monoclonal gammopathies1,2,18 and also when monitoring response to plasmapheresis in patients with light-chain mediated renal failure.21 The involved free light chain concentration or the difference between clonal and nonclonal light chain is recommended for serial evaluation of treatment response rather than the ratio.22

The serum free light chain assays have been fully integrated into the International Uniform Response Criteria in multiple myeloma and also into the hematologic response criteria in AL amyloidosis (Table 2).7,23,24 Serial monitoring with serum free light chain analysis during chemotherapy enables a real-time evaluation of tumor kill due to the short half-life of free light chains and provides a sensitive technique to detect free light chain escape.25,26

The IMWG advocates serial monitoring with serum free light chain analysis for patients with oligosecretory plasma cell disorders including AL amyloidosis.18 For patients with oligosecretory myeloma who have measurable disease with the serum free light chain assays, fewer bone marrow biopsies are required.18 There is insufficient data to recommend routine monitoring with serum free light chain testing for patients with measurable disease by serum or 24-h urine protein electrophoresis.

The serum free light chain assays may prove more reliable than 24-hour urine m-spike determinations in some patients with measurable urinary paraprotein. Siegel and colleagues described a series of patients in which a commercial reference laboratory erroneously reported urine protein electrophoretic results showing evidence of progression of disease when in fact there was no significant change in status. Serum free light chain assays done in parallel were not subject to the same variance avoiding potentially catastrophic errors in management.27

Table 2

Serum Free Light Chain Measurements in the International Uniform Response Criteria in Multiple Myeloma and Hematologic Response Criteria in AL Amyloidosis

 Multiple Myeloma7,24 AL Amyloidosis23 
Stringent complete response 
  • Normal sFLC ratio

  • Bone marrow free of clonal plasma cells by immunohistochemistry or immunofluorescence

 
Not applicable 
Complete response Normal sFLC ratio* 
  • Negative sIFE and uIFE

  • Normal sFLC ratio

  • <5% plasma cells in bone marrow

 
Very good partial response More than 90% decrease in the difference between clonal and nonclonal sFLC levels* Not applicable 
Partial response 50% or more decrease in the difference between clonal and nonclonal sFLC levels* 
  • 50% decrease in SPEP M protein (if >0.5 g/dL at BL)

  • 50% decrease in urine light chains (if >100 mg/day at BL)

  • 50% decrease in clonal sFLC (if >100 mg/L at BL)

 
Progression 25% or more increase from baseline in the difference between involved and uninvolved sFLC levels (absolute increase must exceed 100 mg/L)* From CR: detection of monoclonal protein or abnormal sFLC ratio providing clonal sFLC doubles
From PR: 50% increase in monoclonal protein (to >0.5 g/dL for SPEP, >200 mg/day for UPEP, or >100 mg/L for clonal sFLC) 
 Multiple Myeloma7,24 AL Amyloidosis23 
Stringent complete response 
  • Normal sFLC ratio

  • Bone marrow free of clonal plasma cells by immunohistochemistry or immunofluorescence

 
Not applicable 
Complete response Normal sFLC ratio* 
  • Negative sIFE and uIFE

  • Normal sFLC ratio

  • <5% plasma cells in bone marrow

 
Very good partial response More than 90% decrease in the difference between clonal and nonclonal sFLC levels* Not applicable 
Partial response 50% or more decrease in the difference between clonal and nonclonal sFLC levels* 
  • 50% decrease in SPEP M protein (if >0.5 g/dL at BL)

  • 50% decrease in urine light chains (if >100 mg/day at BL)

  • 50% decrease in clonal sFLC (if >100 mg/L at BL)

 
Progression 25% or more increase from baseline in the difference between involved and uninvolved sFLC levels (absolute increase must exceed 100 mg/L)* From CR: detection of monoclonal protein or abnormal sFLC ratio providing clonal sFLC doubles
From PR: 50% increase in monoclonal protein (to >0.5 g/dL for SPEP, >200 mg/day for UPEP, or >100 mg/L for clonal sFLC) 
*

For patients without measurable disease using serum or urine protein electrophoresis.

BL, baseline; sFLC, serum free light chain; sIFE, serum immunofixation; SPEP, serum protein electrophoresis; uIFE, urine immunofixation; UPEP, urine protein electrophoresis.

Table 3

Prognostic Impact of Abnormal Baseline Serum Free Light Chain Analysis

Diagnosis Prognostic Impact of Abnormal sFLC Analysis 
Multiple myeloma Poor prognosis with involved sFLC ≥750 mg/L34 
 Poor prognosis with involved sFLC ≥856 mg/L22 
AL amyloidosis Shorter survival as involved sFLC concentration increases; median cutoff 152 mg/L32 
Smoldering myeloma Increased risk of progression with sFLC ratio <0.125 or >8.031 
Plasmacytoma Abnormal kappa/lambda ratio predictive of increased risk of progression30 
Monoclonal gammopathy of undetermined significance Abnormal kappa/lambda ratio predictive of increased risk of progression20 
Chronic lymphocytic leukemia Abnormal sFLC ratio predictive of poor survival33 
Diagnosis Prognostic Impact of Abnormal sFLC Analysis 
Multiple myeloma Poor prognosis with involved sFLC ≥750 mg/L34 
 Poor prognosis with involved sFLC ≥856 mg/L22 
AL amyloidosis Shorter survival as involved sFLC concentration increases; median cutoff 152 mg/L32 
Smoldering myeloma Increased risk of progression with sFLC ratio <0.125 or >8.031 
Plasmacytoma Abnormal kappa/lambda ratio predictive of increased risk of progression30 
Monoclonal gammopathy of undetermined significance Abnormal kappa/lambda ratio predictive of increased risk of progression20 
Chronic lymphocytic leukemia Abnormal sFLC ratio predictive of poor survival33 

sFLC = serum free light chain.

Results of urine protein electrophoresis and serum free light chain testing do not correlate well for many reasons. Light chains spill into the urine only after their reabsorption threshold in the proximal convoluted tubules is exceeded. Indeed, normal urine immunofixation and normal urine protein electrophoresis in patients with plasma cell disorders are sometimes observed despite abnormalities in serum free light chain analysis.28,29 In this issue of LABMEDICINE, Siegel and colleagues reported that more than one-half of abnormal urine immunofixations demonstrated intact monoclonal immunoglobulins in addition to free light chains, thus providing another explanation for lack of correlation between serum free light chain results and urine m-protein measurements.27

Prognosis

Serum free light chain analysis has prognostic significance. Prognosis has been shown to be worse among patients with myeloma or AL amyloidosis whose baseline concentrations of clonal free light chains are markedly increased or whose ratio is abnormal at baseline; an abnormal baseline free light chain ratio is a risk factor for progression in patients with smoldering myeloma, MGUS, and solitary plasmacytoma (Table 3).20,22,30–34 Normalization of the serum free light chain ratio after autologous stem cell transplantation has been associated with longer survival in AL amyloidosis.35,36 In contrast, normalization of the serum free light chain ratio as part of the stringent complete response criteria has not been formally validated as a predictor of improved survival.

Conclusions and Future Directions

Clinical applications of the serum free light chain assays have expanded since their first description as a useful diagnostic and monitoring tool in patients with oligosecretory (formerly nonsecretory) multiple myeloma. Assays for free kappa and free lambda light chains are now recommended by the IMWG as an integral component of the screening algorithm for monoclonal gammopathy, for serial monitoring in patients without measurable disease by other methods, as a valuable prognostic tool, and as a component of the stringent complete response criterion in multiple myeloma.

Additional data is needed to explore the potential benefit of measuring urine free light chains in patients with AL amyloidosis whose disease is detectable only by urine immunofixation. Inadequate data is currently available to recommend routine serial monitoring for patients with measurable disease with serum or 24-h urine protein electrophoresis, and additional studies are needed. Finally, the description of several patients with free light chain MGUS10,37 raises the question of the clinical significance of this abnormality. Importantly, evolution of free light chain MGUS to light chain myeloma has been observed in a few patients.38 The absolute risk of progression for patients with free light chain MGUS merits further investigation.

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Author notes

After reading this article, readers should be able to discuss the utility of serum free light chain analysis and its uses in the diagnosis of monoclonal gammopathy, in monitoring patients with oligosecretory disease, and for prognosis of plasma cell dyscrasia.
Chemistry exam 20905 questions and corresponding answer form are located after this CE Update on page 367.