Smoldering multiple myeloma (SMM) is an asymptomatic precursor condition of multiple myeloma (MM). With the introduction of novel therapies that offer high efficiency and improved tolerability, diagnostic criteria have been established to identify patients who may benefit from early therapy. The International Myeloma Working Group (IMWG) and ESMO include a free light chain (FLC) ratio of ≥100 among several accepted criteria and clinical practice guidelines.