Immunochemical measurement of urinary free light chains and Bence Jones proteinuria

To evaluate the characteristics of an immunochemical urinary free light chains (uFLC) test in screening and quantifying Bence Jones proteinuria (BJP) and its potential application in laboratory practice as an alternative to the gold standard method, urine immunofixation (UIFE) and densitometric quantification on 24 h urine sample.

A total of 300 subjects were divided into five groups: controls, patients with BJP Kappa and Lambda present in trace amounts not densitometrically quantifiable, and patients with BJP Kappa and Lambda present in densitometrically measurable quantities. The Wilcoxon-Mann-Whitney test was applied to compare uFLC in controls and BJP trace samples. The correlation between immunochemical and densitometric measurements was assessed using Spearman's correlation coefficient, and agreement was evaluated with Bland-Altman plots. Samples were also stratified by estimated glomerular filtration rate (eGFR) and total urinary proteins (TUP).

Despite significant differences between the median values of uFLC measures in controls and BJP in trace sample groups, using the uFLC upper reference ranges would have led to over 50 % false negative results. Although a strong correlation existed between the two methods, turbidimetry consistently overestimated BJP levels.

uFLC turbidimetric measurements cannot accurately differentiate negative samples from those containing trace BJP, lacking the sensitivity required for clinical use. UIFE has shown greater sensitivity compared to turbidimetry. In monitoring, the systematic overestimation in the quantification of BJP, aggravated in cases of reduced renal function or high TUP concentration, makes it challenging for clinicians to evaluate therapeutic efficacy since the decision thresholds outlined in clinical guidelines are based on densitometric measurements.