Risk-stratification schemes for MGUS according to the 2 major models. In the Mayo Clinic model, the following features are considered to be adverse risk factors: M-protein concentration ≥ 1.5 g/dL, non-IgG isotype, and an abnormal serum FLC ratio (normal reference, 0.26-1.65). Patients with 0, 1, 2, or 3 risk factors are considered low-, low-intermediate, high-intermediate, and high-risk, respectively. The Spanish model uses multiparametric flow cytometry of BM aspirates to differentiate aberrant from normal PCs. PCs characteristically express CD138 and intense (bright) CD38. The features of aPCs include decreased CD38 expression, expression of CD56, and the absence of CD19 and/or CD45. Risk factors for progression are ≥ 95% aPCs/BMPC and DNA aneuploidy.