What would be your next step in workup for a patient with IgG Kappa Monoclonal protein detected on SPEP and free lambda light chains found in the urine, with chronic diarrhea for 3 years and concern for GI amyloidosis?

This is a great question. I think the most important thing when it comes to the consideration of amyloidosis is thinking of it in the first place! 

For patients with MGUS (or myeloma) but with red flag symptoms of amyloidosis, it is important to work these up. The studies recommended may differ based on the suspected organ involved.

In general, the easiest thing to do is a fat pad biopsy. Ideally, enough samples should be taken to allow for congo-red staining and mass spec typing. 

For suspected GI involvement, a colonoscopy/EGD is certainly reasonable with a plan for biopsies to be taken and stained with congo-red (and reflex to mass spectrometry typing to identify the type of amyloid).

It is also important to screen for cardiac amyloid - proBNP, troponins are a helpful first step. Cardiac MRI with contrast to evaluate for late gadolinium enhancement is helpful as well. Neuropathy may be best qualified using EMG/NCS as well, esp. if there are other potential explanations. Also recommend doing a 24-hour urine protein to see if predominantly albuminuria or paraproteinuria, and to quantify it.

The last question, and most important one, is whether one would act on these findings. Renal and cardiac involvement are clear reasons for me to intervene and start treatment. GI involvement can be tricky. If isolated involvement and symptoms are relatively few, it may be reasonable to observe. However, if involvement is more extensive and symptoms are persistent/bothersome, I think it is quite reasonable to start treatment.