TO THE EDITOR: Serum protein electrophoresis (sPEP) and serum immunofixation electrophoresis (sIFE) are the gold standards for diagnosing monoclonal gammopathies (MGs) [1]. However, the PEP and IFE results may vary depending on the method, expertise of the laboratory personnel, and differences between laboratories [2]. Urine protein electrophoresis (uPEP) and urine IFE (uIFE) can be used with high sensitivity [3]; however, the results can be affected by renal function [4]. The free light chain (FLC) assay is an alternative method for diagnosing and monitoring MGs, and has several advantages [3]. First, owing to its short half-life, the FLC assay can be used for real-time monitoring of disease progression or response to treatment in patients with MG [5, 6]. Second, the FLC assay is more sensitive than the PEP and IFE [7]. They are immensely useful, especially in the follow-up of patients with low levels of monoclonal proteins, which account for 20% of MGs [5]. The analytical performance and clinical usefulness of serum FLC (sFLC) assays have been evaluated and compared in previous studies. However, data on urinary FLC (uFLC) assays are limited [8]. Therefore, this study aimed to evaluate and compare the usefulness of sFLC and uFLC assays for diagnosing MGs and other related diseases, and to determine their application in clinical practice.

DATA COLLECTION AND ASSAY METHODS

From June to November 2021, the remaining pairs of serum and 24-h urine samples were collected from patients whose samples were submitted for sPEP, sIFE, uPEP, and uIFE tests as routine examinations. Serum samples were stored at -70°C, thawed, and assayed on the same day. However, urine samples were assayed on the day of collection to prevent the degeneration of urine proteins. We retrospectively reviewed patients' electronic medical records and collected the following data: age, sex, clinical diagnosis, whether the sample was collected at initial diagnosis or follow-up, response to treatment, bone marrow study results (if available), and estimated glomerular filtration rate (eGFR). The eGFR value was calculated using the Modification of Diet in Renal Disease 4-variable formula (isotope dilution mass spectrometry traceable), whereas the body surface area was calculated using the Dubois formula.

Protein electrophoresis and immunofixation were performed using the Sebia Capillarys 2 Flex Piercing System (Sebia, Lisses, France) using the following reagents: Capillarys Protein (E) 6 Kit for sPEP, Capillarys/Minicap Urine Kit for uPEP, and Capillarys Immunotyping Kit for sIFE and uIFE. The detection limit was 0.1 g/dL for sPEP and 2.0 mg/dL for uPEP. sPEP and uPEP results were considered positive if the levels of monoclonal proteins detectable by laboratory personnel were above the detection limit. Total protein and creatinine levels in serum were determined using a colorimetric method, and serum immunoglobulin (sIg) heavy chain and urinary total protein levels were determined using an immuno-turbidimetric method (Cobas c 702 module, Roche Diagnostics, Switzerland). sFLC and uFLC levels were measured using the Freelite assay (The Binding Site Group Ltd, Birmingham, UK), a latex-enhanced immunonephelometric assay measuring free k and l light chains, on an automated Cobas 8000 platform (Roche). The sensitivity of this FLC assay has been reported to be <1 mg/L [9]. The reference interval of FLCs established by the manufacturers was as follows: 3.3–19.4 mg/L for serum k, 5.7–26.3 mg/L for serum l, 0.26–1.65 for the serum k/l ratio, <32.70 mg/L for urine k, <4.99 mg/L for urine l, and 2.04–17.78 for the urine k/l ratio.

This study was approved by the Institutional Review Board of Wonju Severance Christian Hospital (IRB No. CR321321), which waived the requirement for informed consent.

Dow Biomedica (Seoul, Korea), the Korean distributor of the Binding Site, Ltd. (Birmingham, UK), donated the reagents for free light chain (FLC) assays. This study was supported (in part) by a grant from the Yonsei University Future-Leading Research Initiative of 2021 (2021-52-0074).

No potential conflicts of interest relevant to this article were reported.

Table 1 Comparison of serum and urine free light chain values (unit: mg/L) determined by the Freelite assay according to the presence of ND/RR MG (median and interquartile ranges).

Table 2 Comparison of serum and urine free light chain values (unit: mg/L) in non-MG patients determined by the Freelite assay according to the eGFR ranges (median and 95% confidence intervals).

Table 3 Comparison of the analytical performance of serum and urine protein electrophoresis and free light chain assay for diagnosing ND/RR MG (mean and 95% confidence intervals).