Improving diagnostic strategies for plasma cell dyscrasias: a laboratory perspective on Bence Jones proteinuria.

BACKGROUND: The laboratory assessment of Bence Jones proteinuria (BJP) in plasma cell dyscrasias remains analytically challenging. Urinary immunofixation electrophoresis (UIFE) with densitometry is commonly used to identify and quantify monoclonal free light chains, but it is technically demanding and not always feasible. Conversely, nephelometric assays for urinary free light chains (FLCs) and total light chains (TLCs) are automated and quantitative, but they may not specifically reflect monoclonal BJP. In addition, the optimal urine specimen for BJP assessment remains debated, particularly regarding the practical use of spot urine as an alternative to 24-h urine collection. This study compared different analytical approaches for BJP quantification and evaluated the agreement between spot and 24-h urine samples. METHODS: We retrospectively analyzed 123 paired spot and 24-h urine samples from 103 patients with positive screening UIFE. BJP was assessed by UIFE followed by densitometric quantification when technically feasible. Results were compared with nephelometric urinary FLC and TLC measurements. Spot and 24-h urine samples were compared after appropriate normalization, and agreement between methods and specimen types was evaluated using Passing-Bablok regression, Bland-Altman analysis, and non-parametric testing. RESULTS: Densitometric BJP quantification was feasible in 58.5% of spot urine samples and 49.6% of 24-h urine samples, mainly limited by the absence of a measurable peak on the reference track despite visible immunofixed bands and by comigration with intact immunoglobulins. Nephelometric FLC measurements showed a marked positive bias compared with densitometric BJP quantification, exceeding 140% for both Kappa and Lambda light chains, confirming that immunometric FLC assays cannot be considered direct surrogates of monoclonal BJP quantification. Further discrepancies were observed between FLC, TLC, and total urinary protein measurements. No statistically significant differences were found between spot and 24-h urine samples for the investigated parameters. CONCLUSIONS: The choice of analytical method has a major impact on BJP quantification. UIFE remains essential to establish the monoclonal and free nature of urinary light chains, whereas nephelometric FLC and TLC assays may provide complementary but non-interchangeable information. Spot urine, particularly when normalized for creatinine, appears to be a practical alternative to 24-h urine collection in routine laboratory settings.