How does one interpret persistently positive lambda light chains on serum immunofixation but without measurable serum monoclonal protein and a normal light chains and ratio?

I would probably also check a 24 hour urine protein electrophoresis with immunofixation to evaluate if any significant and measurable monoclonal proteinuria that would make me more worried about SMM, MGRS, or amyloidosis. Would check urine protein/creatinine ratio to evaluate protein excretion for t...

Would be curious to hear your thoughts on this. I'm presuming bone marrow biopsy was also negative?

Certain monoclonal antibody drugs, especially if given via subQ, can cause +IFE without a measurable M-spike on SPEP since so low... but the ones I'm aware of (e.g., daratumumab) are all kappa-based...