New free light chain range decreases LC-MGUS

In this issue of Blood, Bertamini and colleagues report improved precision for identifying Black individuals harboring light chain (LC) monoclonal gammopathies (MGs) by using a racially diverse population to establish a new reference interval for the serum free light chain test.¹ 

The study was initiated because of the author’s discovery of an incredibly high rate of light chain monoclonal gammopathy of undetermined significance (LC-MGUS), especially among Black individuals (1074 of 10 035 positive).² That outcome was mainly a result of using the “standard free light chain reference ranges” on measurements of serum from patients in the PROMISE US and PROMISE ZA studies and the Mass General Brigham Biobank (MGBB). The measurements performed using the Freelite assay on an Optilite instrument yielded results that categorized 1074 of 10 035 individuals who self-identified as Black as having LC-MGUS. Although one would expect a ratio of free κ MGUS to free λ MGUS to be about 2:3, in this study 99% of the cases were κ.

Screening of patients with symptoms and signs consistent with multiple myeloma requires a combination of serum protein electrophoresis and immunochemical methods (immunofixation or immunosubtraction) to detect the intact monoclonal immunoglobulins responsible for the 80% of multiple myeloma cases, and serum free light chain measurement to detect the 20% of cases that produce only light chains.^3,4 Mass spectrometry is also now available for detecting these molecules.⁵ Immunofixation and immunosubtraction provide visual proof of the specificity of the intact monoclonal immunoglobin. However, the outcome of the serum free light chain test rests on a reference range and lacks the specificity of visual verification that has resulted in excessive false-positive cases of LC-MGUS, especially in Black individuals.² 

Serum free light chain testing is based on demonstrating a distorted ratio of serum free κ to serum free λ (FLCr), resulting from the increased production of the monoclonal free light chain isotype and sometimes a decrease in the uninvolved isotype.⁶ The ratio is high for κ and low for λ monoclonal gammopathies. Unfortunately, distortions of the FLCr also result from pathophysiologic factors in the patient, such as chronic renal disease and technical issues with the assay itself, and with the genetic characteristics of the population used to create the reference range.^7,8 

The original reference range was established using the Freelite assay (highly purified sheep antibodies attached to latex beads) performed on a BNII instrument in 2002 at the Mayo Clinic using 282 healthy donors.⁷ The 95% reference interval in that study was 0.3 to 1.2, but the authors chose to include the results from all the individuals, resulting in a reference FLCr of 0.26 to 1.65. Although that reference range proved valuable in many subsequent studies, the wide variation of Freelite assay results performed on instruments other than the BNII raised questions of its validity.^8,9 

The questions raised by Bertamini et al are to determine whether the exceptionally high false-positive rate of LC-MGUS in Black individuals resulted from a dearth of Black individuals in the population used to create the original reference range and/or the use of the Optilite instrument for performing the Freelite assay. To achieve this the authors established a new reference range based on race and ancestry by using serum from individuals from the PROMISE US, PROMISE ZA, and MGBB studies, with race determined by self-identification for the individuals in the US PROMISE and ZA PROMISE studies, and by DNA genotyping for those in the MGBB group.

Individuals with impaired renal function were not used in defining the normal FLCr. The population consisted of 3894 White individuals (54%), 3163 Black individuals (44%), and 140 other races (Asian, Asian/Pacific Islander, multiracial, or Native American) (2%). Black individuals had a significantly (P < .0001) higher reference range (median 1.42, 95% confidence interval [CI] 0.84 to 2.47) than White individuals (median 1.17, 95% CI 0.59 to 1.96) or those of other races (median 1.21, 95% CI 0.68 to 2.24).

For the new reference range, the authors first established the central 95% reference values for κ and λ and used only individuals with both free κ and free λ within the new reference intervals. The 6449 individuals in this group provided data for the new FLCr of 0.686 to 2.10.

By using the new FLCr, the diagnosis of LC-MGUS in Black individuals decreased from 10.7% to 0.97%, reflecting both the racial balance of the population used to establish the new FLCr range and the use of the Optilite instrument. The new FLCr range also decreased false positives among White and other races, although to a much lesser extent, establishing the importance of the diverse racial population in establishing the reference range. The new FLCr range of the Freelite assay performed on the Optilite instrument should improve use of the free light chain test in predicting progression of MGUS and smoldering multiple myeloma, confirming stringent complete remission, and following response to therapy.

Although the study focused on large number of false-positive cases of κ LC monoclonal gammopathies using the upper level of the range, especially in Black individuals, the sizable difference between the original value of 0.26 at the lower end of the standard range and 0.686 of the new FLCr range raises concern of false-negative results for λ monoclonal gammopathies in laboratories using the standard range.¹⁰ It would also be more likely in Black individuals because their normal lower end of the FLCr is higher than that of White individuals. Lookback studies are needed to explore this possibility.

Conflict-of-interest disclosure: The author declares no competing financial interests.