CRAB vs. SLiM CRAB
Myeloma, smoldering myeloma, and MGUS were redefined a few years ago to reflect differences in prognosis. MGUS still requires the presence of a serum monoclonal protein in a concentration of 3 g/dL or less, less than 10% plasmacytosis in the bone marrow, asymptomatic status, and absence of end-organ damage as traditionally defined in the acronym CRAB (calcium elevation, renal insufficiency, anemia, or bony disease).
If CRAB is present in a patient with at least 10% plasma cells in bone marrow, that is by definition multiple myeloma warranting treatment. Smoldering myeloma requires at least 10% plasmacytosis in bone marrow and absence of the CRAB criteria. However, in a significant change, ultra–high-risk smoldering melanoma, defined by the acronym SLiM CRAB, is now considered active myeloma and should be treated (Lancet Oncol. 2014 Nov;15[12]:e538-48).
“Traditionally, we waited until CRAB [to define myeloma],” Dr. Mikhael explained. “But if you’re running toward a cliff, I don’t have to wait until you’re falling off to know you’re in trouble.”
The SLiM half of SLiM CRAB consists of 60% or more plasmacytosis in bone marrow, light chains in a kappa-to-lambda or lambda-to-kappa ratio of greater than 100, and MRI showing one or more focal lesions. If a patient is SLiM, with or without CRAB, that is now considered active myeloma warranting treatment.