The urine light chain/glomerular filtration rate (GFR) quotient shows a high sensitivity and specificity to detect cast nephropathy in monoclonal light chain disease.

Background: Cast nephropathy (CN) is associated with a unfavourable outcome in monoclonal light chain (mLC) disease, but also more possible LC‐related renal diseases as well as non‐LC‐related disease can occur. Thus, it is crucial to understand the underlying renal disease. On the other hand, LC can interfere with coagulation preventing kidney biopsy as the gold standard. We sought to develop a non‐invasive algorithm to diagnose CN with a good sensitivity and specificity. Method: We analysed data from patients with mLC disease who underwent kidney biopsy. The patients were classified in 4 groups according the renal histology: CN, AL amyloidosis, light chain deposition disease, and other renal disease. Afterwards, different algorithms were calculated for their sensitivity and specificity. Results: CN showed a significant higher concentration of serum‐free LC and urine LC (LCu), but there was a wide and overlapping range with the other groups. The best accuracy was achieved for a LCu/GFR ratio >2 in patients with lambda LC and either a LCu/GFR > 1 and proteinuria <8 g/24 h or a LCu/GFR > 5 in patients with proteinuria >8 g/24 h in patients with kappa LC. In lambda LC, the sensitivity and specificity for CN was 94% and 90%, respectively; in kappa LC 87% and 81%, respectively. Discussion: In patients with coagulation disturbances due to LC, a non‐invasive algorithm can separate patients with CN from other renal disease in mLC disease. [ABSTRACT FROM AUTHOR]