Impact of renal function impairment on kappa free light chain index

To investigate whether renal function impacts CSF κ-FLC concentration and/or κ-FLC index.

Patients with non-inflammatory neurological diseases were eligible. κ-FLC index was calculated as (CSF κ-FLC/serum κ-FLC)/albumin quotient. Structural equation modeling (SEM) was used to evaluate the direct influence of GFR on serum κ-FLC concentration and albumin quotient (Q_alb), and via these two variables the indirect influence on CSF κ-FLC concentration.

A total of 129 patients with a median age of 65 years and 42 % females were included. κ-FLC index ranged from 0.57 to 3.56 and glomerular filtration rate (GFR) ranged from 17 to 128 mL/min/1.73 m². While a correlation of GFR with CSF κ-FLC concentration was observed (r= -0.52, p<0.001), there was no statistically significant correlation with κ-FLC index (r=0.14, p=0.113). SEM revealed that higher age was associated with lower GFR (β= -0.53), which led to higher serum κ-FLC concentration (β= -0.45) and higher Q_alb (β= -0.17), while CSF κ-FLC concentration increased with serum κ-FLC concentration (β=0.75) and Q_alb (β=0.39), indicating that GFR did not directly influence CSF κ-FLC concentration (RMSEA=0.043).

CSF κ-FLC concentration is not directly affected by renal function. The κ-FLC index compensates for renal function effects by factoring in serum κ-FLC concentration and Q_alb. κ-FLC index can be interpreted without considering renal function.