Interlaboratory study of free monoclonal immunoglobulin light chain quantification

SDS Electrophoresis on Gradient Polyacrylamide Gels as a Semiquantitative Tool for the Evaluation of Proteinuria

Proteinuria is an important sign of kidney diseases. Different protein patterns in urine associated with glomerular, tubular and overload proteinuria may be differentiated using the immunochemical detection of indicator proteins or via urinary proteins electrophoresis. Our aim was to characterize sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) using commercially available 4-20% gradient gels as a method to detect and differentiate proteinuria. Our laboratory-based study used excess urine samples collected for routine diagnostic purposes from adult patients of a tertiary-care hospital, including patients with albumin/creatinine < 30 mg/g and patients with dipstick proteinuria. The limit of albumin detection was estimated to be 3 mg/L. In 93 samples with albumin/creatinine < 30 mg/g, an albumin fraction was detected in 87% of samples with a minimum albumin concentration of 2.11 mg/L. The separation of 300 urine samples of patients with proteinuria revealed distinct protein patterns differentiated using the molecular weights of the detected proteins: glomerular (albumin and higher molecular weights) and two types of tubular proteinuria ("upper" ≥20 kDa and "lower" with lower molecular weights). These patterns were associated with different values of the glomerular filtration rate (median 66, 71 and 31 mL/min/1.72 m², respectively, p = 0.004) and different proportions of multiple myeloma and nephrological diagnoses. As confirmed using tandem mass spectrometry and western blot, the SDS-PAGE protein fractions contained indicator proteins including immunoglobulin G, transferrin (glomerular proteinuria), α1-microglobulin, retinol-binding protein, neutrophil gelatinase-associated lipocalin, cystatin C, and β2-microglobulin (tubular), immunoglobulin light chain, myoglobin, and lysozyme (overflow). SDS-PAGE separation of urine proteins on commercially available 4-20% gradient gels is a reliable technique to diagnose proteinuria and differentiate between its main clinically relevant types.

Multicentered patient-based evidence of the role of free light chain ratio normalization in multiple myeloma disease relapse

INTRODUCTION

The normalization of free light chain ratio (FLCr) has been introduced as a marker of stringent complete remission (CR) of multiple myeloma (MM). There is currently a lack of literature assessing the role of FLCr on MM disease progression and remission status.

PATIENTS AND METHODS

A multicentered retrospective review of 125 patients with MM in CR and various FLCr values was completed. Parameters of interest included patient demographics, FLCr values, complete remission (CR)/relapse status, and time to progression (TTP). The FLCr values were recorded to provide time-dependent findings on the role of FLCr on progression-free survival and overall survival (OS).

RESULTS

The mean follow-up time of 125 patients from five hospitals in the Czech Republic was 31 months. A total of 47.2% of patients relapsed (54 of 125) during the follow-up period. The median TTP of patients with normal FLCr (n = 66) was 54.4 and 40.2 months for patients with abnormal FLCr (n = 59) (P = 0.217). None of the patients reached median overall survival regardless of FLCr values (P = 0.821). In the subgroup of newly diagnosed patients after upfront autologous stem cell transplantation (ASCT), there were 55.6% of patients (35 of 63) with normal FLCr and 44.4% (28 of 64) with abnormal FLCr. A total of 34.9% of patients (22 of 63) relapsed in this subgroup. Within the abnormal FLCr patients, a median TTP was 56.3 months, but no median TTP was reached among the normal FLCr patients (P = 0.746). Median OS in patients with normal (nFLCr) and abnormal FLCr (aFLCr) was not reached (P = 0.787).

CONCLUSION

We did not observe any benefit from FLCr normalization in CR in myeloma patients in terms of progression-free survival or overall survival.

Defining the impact of individual sample variability on routine immunoassay of serum free light chains (sFLC) in multiple myeloma

Serum free light chain (sFLC) measurement has gained widespread acceptance and is incorporated into various diagnostic and response criteria. Non-linearity and antigen excess are the main causes of 'variability' in the measurement of sFLC using immunoassay, but the impact of these on measurement has been unclear. We performed a retrospective evaluation using a dilutional strategy to detect these phenomena. A total of 464 samples in 2009 and 373 samples in 2010 were analysed for sFLC. Non-linearity was detected in both high and apparently normal sFLC. Major non-linearity of more than twofold is common in high kappa (20·2%) and lambda (14·1%). It is less common in samples with apparently normal levels - kappa (6·4%) and lambda (9·5%). 9·4% of kappa and 15·5% of lambda showed antigen excess at screening dilutions. 34·4% of the samples had either non-linearity or antigen excess. We conclude that significant measurement variability is common in the measurement of sFLC. There is currently no reliable technique to detect non-linearity phenomena unless a serial dilution strategy is applied to every analysis. We recommend that laboratories routinely reporting sFLC results for clinical services need appropriate strategies for addressing these issues. Clinicians should be aware of these limitations in interpretation of sFLC assay for individual patients. Future guidelines should adopt action thresholds which are grounded firmly in test performance parameters.