When both serum and urine tests are available, it is clinically reassuring to have two separate tests. Clearly, samples do occasionally get incorrectly analysed or mislabelled, so any discrepancy between sFLC and urine BJP tests will direct the laboratory to further investigate . In addition, supporting evidence from either FLC test can be helpful when making a diagnosis or changing treatment, and it should be remembered that, in the context of a stem cell transplant in MM patients, for example, the additional cost of performing both serum and urine tests is inconsequential.
If a choice has to be made between serum or urine tests, then the use of serum is clearly preferable for the many reasons given above and summarised in Table 24.2 . For example, Dejoie et al.  state, "We believe there is now sufficient evidence and experience to propose that sFLC analysis is the method of choice for response evaluation in LCMM patients, whereas urine exploration may remain a rational practice to assess total proteinuria and, together with serum creatinine measurements, monitor renal function in these patients." A recent editorial by Mollee and Tate  concludes that "…the time has come for the [sFLC] assay to be the preferred tool to monitor myeloma not measurable by [SPE]. This will greatly facilitate the monitoring of myeloma, enable diagnosis of light chain escape, and leave only the occasional patient who requires [24-hour UPE] monitoring."
|Serum versus urine measurements|
|Easy to collect||Difficult to collect|
|κ/λ ratio less affected by renal function||Renal function affects levels|
|Easily analysed||Samples may need concentrating|
|Easily stored||More difficult to store|
|More frequently abnormal in NSMM and AL amyloidosis||Less frequently abnormal|
|More sensitive for monitoring patients|| Less sensitive for monitoring patients|
Table 24.2. Summary of clinical and analytical comparisons of sFLC and urine electrophoresis tests .