Monoclonal gammopathy is, by definition, the presence of a monoclonal immunoglobulin ("M-spike") and/or abnormal free light chains ("FLCs") in blood, urine or both, produced by clonal plasma cells. A sufficiently high M-spike or sufficiently high/low K/L FLC ratio raises the risk level for progression to SMM or MM, and should prompt a BMB - in my case, a very low M-spike, but a K/L ration of effectively 0. If the BMB shows >10% plasma cells, then the diagnosis becomes either SMM or MM, depending on presence of symptoms and/or signs of organ damage. In other words, a BMB is not necessary for a diagnosis of MGUS. This is a very general account; there are multiple levels of complexity.

NB: I am not a physician, although I played one in various contexts (one of which was not television, for those of you who recall the reference) before I retired.