Do you recommend routinely getting periodic 24-hour urine monoclonal protein study for patients with active multiple myeloma?

In short, no.
I like to perform at the time of diagnosis and then at key points in the patient's journey if it was grossly abnormal (at the time of a bone marrow biopsy, for example).
Patients definitely do not enjoy it. It does allow for IMWG response assessment, which is mandated for patients on trial but can be helpful in certain circumstances in the clinical context (esp. if there is predominantly albuminuria).
Also, when we do it, it is just UPEP with immunofixation. I do not send urine ligh...

Lugging a gallon jug of urine to a clinic visit is the kind of minor indignity that is easy for us to discount, but that can weigh heavily on some patients.

Urine studies are occasionally useful to differentiate light chain cast nephropathy or amyloidosis from other forms of kidney injury. But in general, urine and serum paraprotein levels rise and fall in parallel, so a 24-hour urine provides no additional information beyond what we already get from blood tests.

It is surprising how many places...

Excellent question - the only addition I'll make to @Dr. First Last's excellent answer is that these tests rarely even change IMWG response criteria either. We've studied this here (ASH abstract below, manuscript under review):

Natsuhara et al., Blood 2022

And hopefully, a registry-based analysis of this topic on the way soon. Apart from patients with significant albuminuria and/or some evidence of MGRS (e.g., AL amyloidosis), I don't think there is any benefit to 24-hour urine testing - just ma...

Completely agree with @Dr. First Last and @Dr. First Last (thanks for that reference).

I have always been curious why it still is included in the response criteria.

The two places where I have found the value of doing the 24hr monoclonal protein:

1. MM where FLC ratio >100 is the only myeloma defining event but urine monoclonal protein is low (suggesting likely no/low risk of cast nephropathy) to follow these patients as smoldering MM (Visram et al., PMID 35190659).
2. Suspicion for development of amylo...