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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 [528]. When both serum and urine tests are available, it is clinically reassuring to have two separate tests. Clearly, samples do occasionally get incorrectly analysed, mislabelled or misplaced, so supporting evidence for making a diagnosis or changing treatment is always helpful. 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.

Serum versus urine measurements
sFLC Urine electrophoresis
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 [528].

Questions

  1. Is there any remaining role for urine FLC tests when screening for monoclonal gammopathies?
  2. Why are patients with excess monoclonal κ sFLCs more likely to have positive urine results?
  3. Which is the more sensitive for detection of residual disease, serum or urine FLC analysis?

Answers

  1. The IMWG recommend that in the context of screening for the presence of MM or related disorders, the sFLC assay in combination with SPE and sIFE yields high sensitivity and negates the requirement for 24-hour urine studies for diagnoses other than AL amyloidosis, which requires uIFE in addition to the two serum tests. The IMWG also state that urine FLC immunoassays are not recommended (Chapters 23 and 25).
  2. Monomeric κ FLC molecules filter through the glomeruli more readily than dimeric λ molecules (Section 24.2).
  3. All published studies have found that sFLC analysis is more sensitive for monitoring patients than urine electrophoresis (Section 24.8).
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References

    • 1. Carr-Smith HD, Mead GP, Bradwell AR. Serum free light chain assays as a replacement for urine electrophoresis. Haematologica 2005;90:PO404a