Overview - The clinical importance of serum free light chain analysis

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SECTION 1 - Overview



The serum free light chain (Freelite®) assay is:
  1. Used for diagnosis, monitoring and prognosis in plasma cell dyscrasias and is widely accepted in national and international guidelines.
  2. Able to replace urine testing in a multiple myeloma (MM) diagnostic screen and avoids urine compliance issues.
  3. An essential and key component for the risk stratification of MGUS and its successful management.
  4. Complemented by the recently developed intact immunoglobulin assay (Hevylite®).

For more than 150 years, the presence of Bence Jones protein (immunoglobulin free light chains [FLCs]) in the urine has been an important diagnostic marker for multiple myeloma (MM). Indeed, it was the first in vitro cancer test to be used, a century before any others [1]. In the last few years, interest in FLCs has undergone a renaissance with the development of serum tests for free kappa (κ) and free lambda (λ) light chains. The clinical importance of these tests continues to grow as new applications for their use emerge [2]. By way of comparison, the management of diabetes mellitus was hugely improved when blood replaced urine for glucose analysis.

From a physiological viewpoint, blood tests for small molecular weight proteins have clear advantages over urine tests. Serum FLCs are rapidly cleared through the renal glomeruli with half-lives of between 2 and 6 hours before being metabolised in the proximal tubules of the nephrons. Under normal circumstances, little protein escapes to the urine so serum FLC concentrations have to increase many-fold before absorption mechanisms are overwhelmed (Chapter 3). Hence, urinalysis is a fickle witness to changes in FLC production. Conversion to a serum test has provided clarity in assessing disease processes that were previously hidden from view.

Serum concentrations of FLCs are dependent upon the balance between production by plasma cells (and their progenitors) and renal clearance. When there is increased polyclonal immunoglobulin production and/or renal impairment, both κ and λ FLC concentrations can increase 30-40 fold. However, the relative concentrations of κ to λ (i.e. the κ/λ ratio) remain unchanged, or only slightly increase (Chapter 20). In contrast, tumours produce a monoclonal excess of only one of the light chain types, often with bone marrow suppression of the alternate light chain, so that κ/λ ratios become highly abnormal. Accurate measurement of κ/λ ratios underpins the utility of serum FLC immunoassays and provides a numerical indicator of clonality [3]. Urine κ/λ ratios are not as dependable because the non-tumour light chain production is too low to guarantee consistent passage through the nephrons. In contrast, electrophoretic tests can only be used to quantify monoclonal light chain peaks because they are not sensitive enough to identify the non-tumour FLC concentrations.

Early clinical studies with serum FLC tests were performed in patients with Bence Jones (light chain) multiple myeloma. In two studies, on 252 sera taken at clinical presentation, highly abnormal serum FLC concentrations were found in every case [4][5]. Furthermore, during chemotherapy, urine tests frequently normalised whereas serum tests remained abnormal, indicating their increased sensitivity for detection of residual disease. In this patient group, analysis of urine can now be replaced by serum FLC tests. This is particularly helpful for frail, elderly patients because 24-hour urine samples are difficult to collect and results may be unreliable [6].

Around 3-4% of patients with multiple myeloma have so-called nonsecretory disease (Chapter 9). By definition these patients have no monoclonal proteins when assessed by serum and urine electrophoretic tests. Nevertheless, in a study by Drayson et al. [7], serum FLC tests identified monoclonal proteins in 70% of 28 such patients. A study by Katzmann et al. [8] found that all of 5 patients with untreated nonsecretory myeloma had abnormal FLC concentrations. It is apparent that these patients’ tumour cells produced small amounts of monoclonal protein. Their serum FLC concentrations were below the sensitivity of serum electrophoretic tests and below the threshold for clearance into the urine. Importantly, these patients can now be closely monitored by serum FLC tests rather than repeated bone marrow biopsies or whole body scans.

Approximately 20% of all patients with myeloma have light chain or nonsecretory myeloma. Among the remaining patients, those who produce intact monoclonal immunoglobulins, FLCs are abnormal in 96% at disease presentation [9] (Chapter 10). Interestingly, the serum concentrations of FLCs and intact monoclonal immunoglobulins are not correlated (R2 < 0.1). Monoclonal serum FLCs are therefore independent markers of the disease process and have been shown to have prognostic value [10]. This is of potential clinical use when the tumour produces large amounts of FLCs and small amounts of intact monoclonal immunoglobulins. Thus, patients who are in apparent remission, as assessed by measurement of intact monoclonal immunoglobulins, may still have residual disease as judged by elevated monoclonal FLCs. Using a similar argument, when these patients relapse, FLC concentrations may increase first [11]. Free light chain “breakthrough” is estimated to occur in 2 - 5% of patients who relapse after modern, intensive treatment [12] (Chapter 12.7).

An additional feature of FLC molecules is that, in contrast to intact immunoglobulins, they are frequently nephrotoxic (Chapter 13) . Indeed, “myeloma kidney”, which presents as acute renal failure, occurs in approximately 10% of patients [13]. The life expectancy of these patients is then often significantly reduced. Myeloma kidney is particularly common in light chain only disease, but in many patients with intact monoclonal immunoglobulins the serum FLC concentrations are >1,000mg/L (50-100 times normal) and there is associated renal damage. This is characteristic of patients with IgD multiple myeloma but is also apparent in 10-15% of IgG- and IgA-producing patients. The FLC assays allow assessment of the pre-renal load of monoclonal light chains in virtually all of these patients.

There is early evidence that treatment should be aimed to rapidly reduce serum FLC concentrations in order to prevent further renal damage [14]. Furthermore, rapid removal of nephrotoxic FLCs, using high cut-off dialysers, can lead to renal recovery [13][15] (Chapter 20.4). This important new development, when used in combination with aggressive chemotherapy, should lead to significant increases in survival in this serious disease.

One particularly interesting aspect of serum FLCs is their short half-life in the blood (κ 2-4 hours, λ 3-6 hours) (Chapter 12.4). This is approximately 100-200 times shorter than the 21-day half-life of IgG molecules. Hence, responses to treatment are seen in “real time”. This is apparent from the good correlation between bone marrow assessment of disease status and FLC concentrations but poor correlation with IgG concentrations [9]Thus, FLC concentrations allow more rapid assessment of the effects of chemotherapy than do monoclonal IgG or IgA concentrations. The impact of this phenomenon is likely to be considerable: for instance, the resistance of patients to particular drugs or drug combinations can be observed quickly and alternative treatments considered. In addition, the short half-lives of FLCs often allow assessment of complete tumour responses after one or two cycles of chemotherapy and before stem cell transplantation.[16] Thus, IgG with its 21-day half life is a slow marker of treatment responses whereas FLC analysis allows more accurate assessments [17][18][19].

Serum FLC tests are also of considerable importance in AL (primary) amyloidosis (Chapter 15). Characteristically, light chain fibrils are deposited in various organs and tissues and lead directly to disease. The origin of the fibrils is monoclonal FLCs produced by a slowly growing clone of plasma cells. While concentrations of these are often insufficient for measurement by serum electrophoretic tests, serum FLC assays provide quantification of the circulating fibril precursors in 88-98% of patients [20][5]. Furthermore, the tests allow assessment of treatment responses and disease relapses that, in turn, correlate with survival. As stated by Dispenzieri et al. [21] from The Mayo Clinic:

"Introduction of the serum immunoglobulin free light chain assay has revolutionized our ability to assess hematological responses in patients with low tumor burden".

Further support for the role of serum FLC analysis in AL amyloidosis was provided by Katzmann et al [8]. The combination of serum FLC and serum immunofixation electrophoretic tests identified 109 of 110 patients at diagnosis. FLC analysis alone identified 91% of the patients while immunofixation electrophoresis identified only 69%, and urinalysis failed to identify the sole patient that was normal by both serum tests. A similarly high sensitivity of the FLC assays has been reported for light chain deposition disease [3][8].

Many national and international guidelines, including those from the UK and USA, and the Consensus Opinion in AL Amyloidosis support the use of serum FLC measurements[22][23][24] (Chapter 25). Reduction of the κ/λ ratio to normal, alongside the intact monoclonal immunoglobulins, is the benchmark for complete or stringent complete serological responses to therapy in these and other monoclonal diseases.

An emerging role for serum FLC analysis is in the assessment of risk of progression in individuals with monoclonal gammopathies of undetermined significance (MGUS) (Chapter 19). These are premalignant states that progress to multiple myeloma, AL amyloidosis or other plasma cell dyscrasias at a rate of approximately 1% per year. Rajkumar et al [25] have shown that the presence of an abnormal serum FLC κ/λ ratio is a major independent risk factor for progression. In particular, the 40% of MGUS patients with low levels of IgG M-spike (<15g/L) and normal κ/λ ratios had a 21-fold lower risk of progression than patients with an M-spike of >15g/L, abnormal κ/λ ratios and non-IgG immunoglobulin class.

The predictive value of baseline serum FLCs for progression or prognosis extends to most monoclonal gammopathies. This has been shown for asymptomatic myeloma [26], solitary plasmacytoma [27], AL amyloidosis [28], Waldenström’s macroglobulinaemia [29] and, most recently, multiple myeloma [30]. Indeed, initial serum FLC concentrations are independent of serum albumin and β2-microglobulin as a risk factor for myeloma progression, suggesting that FLCs should be added into the current international staging system (ISS) for multiple myeloma [10].

The high sensitivity of serum FLC immunoassays for tumour detection indicates that they have a role in the initial screening for plasma cell dyscrasias. Historically, symptomatic patients were assessed using serum and urine protein electrophoretic tests. Since urine is frequently unavailable, it is logical to add serum FLC analysis to current test protocols (Chapter 24.7). In a study of 1,003 consecutive unknown samples by Bakshi et al. [31], serum FLC analysis identified an additional 16 patients with monoclonal proteins above the 39 detected by serum capillary zone electrophoresis. B-cell/plasma cell tumours were present in 9 of the 16, including 2 with light chain and 1 with nonsecretory multiple myeloma. Confirmation of the sensitivity of a serum panel was provided by Katzmann et al. [32], who showed that among 428 patients positive for urine monoclonal immunoglobulins, all except one were identified by serum tests. The one sample that was only urine positive was of no clinical consequence. Other studies have demonstrated similar benefits and, in each, little or no gain was achieved from urine tests [33][34][35].

All of these findings indicate that the combination of serum electrophoresis, serum immunofixation, and FLC analysis is a clinically sensitive strategy for identifying patients with MM and, to this end, was recently recommended by the International Myeloma Working Group[36]. For clinically suspected amyloidosis, it was recommended that urine immunofixation be added to the panel[36]. As a consequence, serum free light chain analysis for assessing monoclonal gammopathies is being widely adopted. However, the question of whether serum is additional to or can replace urine tests for monitoring patients remains unresolved.

FLC concentrations have also been assessed in cerebrospinal fluid samples (CSF) (Chapter 22). In a study by Fischer et al. [37] kappa concentrations provided information comparable with oligoclonal band measurements. They concluded that CSF kappa FLC measurements may be a useful diagnostic procedure for detecting, and potentially monitoring, intrathecal immunoglobulin synthesis.

In summary, serum FLC tests are assuming an important role in the detection and monitoring of monoclonal gammopathies, bringing benefits to the multitude of patients with plasma cell dyscrasias. The increasing clinical use of this new approach is clearly apparent from the number of recent reviews [38][39][40][41][42].

Back to Contents Page Chapter 1


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  17. Pratt G, Mead GP, Godfrey KR, Hu Y, Evans ND, Chappell MJ, et al. The tumor kinetics of multiple myeloma following autologous stem cell transplantation as assessed by measuring serum-free light chains. Leuk Lymphoma 2006;47:21–8 PMID: 16321823
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  20. Lachmann HJ, Gallimore R, Gillmore JD, Carr-Smith HD, Bradwell AR, Pepys MB, Hawkins PN. Outcome in systemic AL amyloidosis in relation to changes in concentration of circulating free immunoglobulin light chains following chemotherapy. Br J Haematol 2003;122:78–84 PMID: 12823348
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  22. Bird JM, Cavenagh J, Samson D, Mehta A, Hawkins P, Lachmann H. Guidelines on the diagnosis and management of AL amyloidosis. Br J Haematol 2004;125:681–700 PMID: 15180858
  23. Durie BG, Harousseau JL, Miguel JS, Blade J, Barlogie B, Anderson K, et al. International uniform response criteria for multiple myeloma. Leukemia 2006;20:1467–73 PMID: 16855634
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  28. Dispenzieri A, Lacy MQ, Katzmann JA, Rajkumar SV, Abraham RS, Hayman SR, Kumar SK, Clark R, Kyle RA, Litzow MR, Inwards DJ, Ansell SM, Micallef IM, Porrata LF, Elliott MA, Johnston PB, Greipp PR,Witzig TE, Zeldenrust SR, Russell SJ, Gastineau D, Gertz MA. Absolute values of immunoglobulin free light chains are prognostic in patients with primary systemic amyloidosis undergoing peripheral blood stem cell transplantation. Blood 2006; 107 (8): 3378-3383. PMID: 16397135
  29. Itzykson R, Le Garff-Tavernier ML, Katsahian S, DiemertM-C, Musset L, Leblond V. Serum Free Light Chain Elevation Is Associated with a Shorter Time to Treatment in Waldenstrom’s Macroglobulinemia. Haematologica 2008;93:793-794. PMID: 18450739
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  34. Abadie JM, van Hoeven KH, Wells JM. Are renal reference intervals required when screening for plasma cell disorders with serum free light chains and serum protein electrophoresis? Am J Clin Pathol 2009;131:166-71. PMID: 19141376
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