Diagnostic Performance of the New Free Light Chain Ratio in Systemic Amyloidosis

Background: Detection of monoclonal components (MCs) using serum and urine immunofixation (IFE) and free light chain measurement is a critical early step in diagnosing cardiac amyloidosis. Patients with MCs are referred for biopsy-based diagnostic work-up. New reference ranges for the free light chain ratio (FLCR), adjusted for age and estimated glomerular filtration rate, have been proposed. Objectives: The aim of this study was to compare the diagnostic performance of the new vs the conventional FLCR in patients with light chain (AL) amyloidosis and wild-type transthyretin (ATTRwt) amyloidosis. Methods: The analysis included 1,705 patients with AL amyloidosis and 675 with ATTRwt amyloidosis. Results: In the AL cohort, 44 patients (3%) had negative results on serum and urine IFE at diagnosis. Among these, 13 patients had normal conventional FLCRs and 15 had normal new FLCRs, with no significant difference in diagnostic sensitivity (70.4% [95% CI: 55.8%-82.5%] vs 65.9% [95% CI: 50.0%-79.5%]; P = 0.82). Overall diagnostic sensitivity for MC detection was similar between the new and conventional FLCRs (99.2% vs 99.1%). Among patients with ATTRwt amyloidosis and negative serum and urine IFE results, 156 (26.5%) had abnormal FLCRs using the conventional cutoff, compared with only 12 (2%) using the new FLCR. At hematologic response assessment in AL amyloidosis, concordance in the definition of complete response between the 2 FLCR methods was 94.9% (95% CI: 93.4%-95.9%). Conclusions: The new FLCR demonstrates equivalent diagnostic sensitivity in AL amyloidosis and can be integrated into complete response criteria. In patients with suspected ATTRwt amyloidosis, it significantly reduces the proportion of FLCR-only abnormalities, thereby limiting the need for biopsy confirmation.