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6.4 Freelite urine reference intervals

Chapter 6

Bradwell et al. [81] measured κ and λ concentrations in early morning urine samples from 66 normal individuals using Freelite immunoassays (Figure 6.5 and Table 6.3). When compared to sFLC data from the same study, the range of urine FLC concentrations was much wider than for serum, and κ/λ urine FLC ratios were more variable. Presumably, this reflects differences in renal handling, urine dilution and mucosal secretion of FLCs between individuals.

Normal adult urine Mean 95% reference range
κ FLC
5.4 mg/L (± 4.95)
0.39 - 15.1 mg/L
λ FLC
3.17 mg/L (± 3.3)
0.81 - 10.1 mg/L
κ/λ ratio
1.85
0.46 - 4.0

Table 6.3. Mean values (± standard deviation) and ranges for FLC concentrations and κ/λ ratios in early morning urine samples from 66 normal individuals [81].

A wide range of normal urine FLC concentrations was similarly observed by Snyder et al., who established a 95% reference range for the urine Freelite ratio of 1 - 19 using 91 healthy adult donors [166]. The authors attributed the relatively poor diagnostic sensitivity of the urine FLC assay (80%) to the wide reference ranges and a high background of polyclonal FLC in the urine. It is important to note that international guidelines do not recommend the use of urinary FLC immunoassays [167]. Arguments in favour of serum over urine FLC assays are further discussed in Chapter 24.

Questions

  1. Why is the normal κ/λ ratio inverted in serum compared with urine?
  2. Why do κ/λ sFLC ratios increase slightly in patients with severe renal impairment?

Answers

  1. Because the smaller monomeric κ molecules are cleared faster by the kidneys and enter the urine more readily than dimeric λ molecules (Section 6.1).
  2. Because FLC removal in renal impairment becomes dependent upon the reticuloendothelial system which removes κ and λ FLC at the same rate. Serum levels begin to reflect production rates and κ concentrations therefore increase relative to λ (Section 6.3 and Chapter 3).
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