Novel Approach to Rule-Out Unnecessary Urine Bence Jones Protein Testing: A Serum Free Light Chain Algorithm

<b>Background/Objectives:</b> Bence Jones proteins (BJPs) are monoclonal immunoglobulin free light chains (FLCs) that appear in the urine of patients with plasma cell disorders, including multiple myeloma (MM), Waldenström's macroglobulinemia (WM), or light chain amyloidosis (AL). Their presence can provide valuable information about disease progression and treatment efficacy. These proteins are typically detected through a 24-h urine collection, as recommended by clinical guidelines. However, this method can be inconvenient for both patients and laboratory personnel due to its time-consuming nature and the potential for collection errors. We propose an algorithm based on serum FLC (sFLC) to rule out the presence of BJPs and diminish the need for urine testing. <b>Methods</b>: A retrospective data analysis of 268 serum and urine samples from 44 patients with MM was performed, and cutoffs were established to predict BJP absence: total urine protein (0.115 g/L), sFLC κ/λ ratio (>0.82 λ monoclonality and <1.99 κ monoclonality), and difference of involved-uninvolved FLC (dFLC; <11.93 mg/L). A subsequent algorithm validation was performed in 716 samples from patients who underwent the same testing in routine 2023 other laboratory activity. <b>Results</b>: The validation of these cutoffs to rule out the presence of BJP showed that, if the protocol based on the sFLC κ/λ ratio and dFLC had been applied, 42% of the urine studies would have been avoided, achieving a sensitivity of 93.9% and a false negative rate of 6.11%. <b>Conclusions</b>: We propose a laboratory work protocol that would allow for the avoidance of almost half of the 24-h urine studies based on sFLC measurement, a faster and more objective alternative to urine analysis for screening out the presence of BJP, with a good sensitivity and a low false negative rate.