Kappa free light chain as a predictor of tubular damage in first morning urine

Urine alpha-1-microglobulin protein is a sensitive biomarker for detecting damage in the proximal renal tubules. Free kappa light chains may also be elevated in tubular damage of any origin. The aim of this study was to evaluate whether the detection of free kappa light chains in urine could show earlier tubular damage than alpha-1-microglobulin.

A total of 330 analyses performed between 2020 and 2024 were selected, corresponding to patients who had an estimated glomerular filtration rate between 8-131 mL/min/1.73 m². Those samples that were normal in first morning urine were selected and the alpha-1-microglobulin adjusted for creatinine was calculated. Taking into account the total population, 2 groups were formed: group with normal alpha-1-microglobulin adjusted for creatinine in first morning urine -NA1MG- and group with elevated alpha-1-microglobulin adjusted for creatinine in first morning urine -EA1MG-.

The third quartile of kappa free light chain/creatinine in NA1MG was 15.32 (mg/L)/creatinine (g/L), and 39 samples had a higher value. All of them were within the interquartile range of the kappa free light chain/creatinine value of the EA1MG group (17-98.04 [mg/L]/creatinine [g/L]). In turn, 36 samples from the EA1MG group had a kappa free light chain/creatinine value < 15.32 (mg/L)/creatinine (g/L), and the remaining 126 urine samples from this group had elevated levels of both parameters, alpha-1-microglobulin and kappa free light chains.

Free kappa light chain/creatinine ratio (mg/L)/(g/L) > 15.32 in first morning urine shows tubular damage even with normal alpha-1-microglobulin/creatinine values (<7.08 alpha-1-microglobulin [mg/L]/creatinine [g/L]). Tubular damage would begin with isolated elevation of alpha-1-microglobulin or kappa light chains, with earlier elevation of the latter observed in younger patients. Both are more elevated when tubular damage is more severe.