• Normal renal tubular metabolism prevents urine excretion of significant amounts of monoclonal FLCs. This mechanism ensures that some patients have abnormal sFLCs but normal urine.
  • Urine is not usually supplied at initial patient screening, whereas a serum sample is available.
  • Methodology for urine electrophoresis varies considerably between laboratories, and interpretation can be challenging.
  • Further benefits of sFLC analysis over urine tests include improved clinical sensitivity and a more rapid reporting turnaround time.

Most of the issues relating to the use of serum versus urine for the measurement of free light chains (FLCs) have been detailed individually in preceding chapters. This chapter summarises these arguments and attempts to provide a coherent discussion of the relative merits of serum versus urine testing. Some laboratories continue to favour urine over serum measurements: this chapter aims to persuade them otherwise. An analogy with diabetes mellitus is helpful: 50 years ago, all patients were monitored using urine glucose tests, whereas now they are monitored using blood glucose due to its overwhelming clinical advantages. As glucose and FLCs are handled in a similar manner by the kidneys, similar benefits accrue from testing serum over urine for FLC analysis.

"If free light chains are in the urine they are always in the serum first".