Patients with light-chain amyloidosis and low free light-chain burden have distinct clinical features and outcome.

The validated criteria of hematologic response in light-chain (AL) amyloidosis are based on the measurement of circulating free light chains (FLCs). Patients with a difference between involved and uninvolved FLC (dFLC) <50 mg/L cannot be assessed for response and are excluded from clinical trials. We compared the clinical characteristics and outcome of 203 newly diagnosed patients with dFLC <50 mg/L (low dFLC) with 866 patients with measurable dFLC (high dFLC) evaluated between 2004 and 2015. Heart involvement was significantly less common and less advanced in the low-dFLC group (43% vs 83% and Mayo stage III 45% vs 15%, both P < .001), whereas renal involvement was more frequent (77% vs 63%, P < .001) and more severe (renal stage III 26% vs 18%, P = .001). Overall survival (OS) was significantly better in the low-dFLC group (median 117 vs 21 months, P < .001), whereas no difference was seen in renal survival (RS). Within each Mayo stage, patients with low dFLC had a longer survival. In the low-dFLC group, complete response was associated with a significant advantage in OS (median not reached vs 117 months, P = .005) and with a better RS. A reduction in dFLC after therapy of <10 mg/L was associated with a better OS and RS in patients with at least a dFLC >20 mg/L baseline. Nineteen percent of newly diagnosed patients with AL amyloidosis have low dFLC and had a better outcome. Hematologic response assessed with adapted criteria predicts OS and RS in these patients, who can thus be assessed for response and included in clinical trials with appropriate stratification.