Do you always start treatment for patients who meet diagnostic criteria for multiple myeloma by SLiM criteria alone?

No, I definitely do not always start treatment for patients who meet the so-called myeloma defining events. These are guidelines, like the Pirate Code.

Sixty percent plasma cells mean reaching CRAB criteria in 2 years in 95% of patients, greater than one 5 mm MRI abnormality means CRAB criteria in 75%, and an abnormal free light chain ratio >100 means CRAB criteria in 64-79% in 2 years. We have all sorts of opinions about how we might see a 75% risk in 2 years, but I always talk to my patients about how they view this risk.

The criteria we seem to argue about the most is the free light chain ratio, whether by free lite or Sebia. Accuracy is improved when we combine a UPEP > 200 mg/day with a free light chain ratio ≥ 100. Still not perfect, ...

I agree with @Dr. First Last - the expanded diagnostic criteria have no direct clinical impact and do not carry quite the same weight as the classic CRAB criteria. I have several elderly patients whose light chain ratios surpassed 100, and who after a balanced discussion opted to defer treatment. They have generally remained stable with good quality of life, and in some cases, the ratio has fluctuated downward without treatment.

I would stress that these are individualized cases. I still regard the IMWG definitions as the standard of care criteria to initiate treatment, and a decision to do otherwise can only be made collaboratively with a properly informed patient.