What is your threshold to diagnose light chain MGUS with only a slight elevation in the light chain ratio, in a patient without other CRAB symptoms and no M spike?
Mild increase in kappa light chains can be a result of antigenic drift seen with the assay, in renal dysfunction both can go up but kappa may go up a bit more skewing the ratio. If a 24-hour urine with electrophoresis and immunofixation is negative and the rest of the work up is negative, would continue to follow, recheck in 3-6 months, and if no change, repeat in a year and then disregard. Assuming serum iFE is negative.
Marginally abnormal light chain ratio, usually kappa-predominant, is an extremely common referral in my practice, and my intuition has long been that most of these patients are well and don't need further workup or even long-term monitoring. This intuition is now supported by published data from the Icelandic iStopMM study, at least for patients with renal insufficiency:
The proposed intervals classify kappa/lambda ratios up to 3.38 as normal, depending on the degree of renal dysfunction. This considerably reduces the number of patients who are diagnosed with kappa light chain MGUS and told they need indefinite laboratory monitoring.
Even in patients who clearly meet the criteria for MGUS, I do not routinely perform a bone marrow biopsy at the initial visit. Triggers to do so include anemia, hypercalcemia, or renal insufficiency without an obvious cause; high monoclonal burden; or progressive increase in monoclonal protein over time.
In the iStopMM cohort - light chain ratio new definition based on GFR when 45-60: 0.46 - 2.62, GFR 30-45: 0.48 to 3.38 and GFR<30 0.54 - 3.30.
Higher age >70 got a new definition of higher reference range of 0.46 to 2.59.
If elevated based on new reference ranges - then it is light chain MGUS but it would be considered low risk with no m spike.
I use the following guidelines for bone marrow biopsy.
If M spike >1.5 mg/dL or has non-IgG M spike or elevated light chain involved/uninvolved >8 for light chain MGUS
In the absence of CRAB and just slight elevation, would not consider a bone marrow. Would use the iStopMM.com formula to assist in the decision.