My own understanding, ackibyfobia, is that the Bence Jones urine test is only relevant in the case of suspected amyloidosis, otherwise the blood test is the gold standard for myeloma investigations for a number of reasons. Having said that I’ve had two 24 hour Bence Jones urine tests and I never did find out what happened to the second one, but I have my suspicions. Taking a 24 hour urine sample into your GP or the hospital invokes a strange look from the recipient. It was even funnier when I asked my GP for a sample bottle as I knew I’d need an awful lot of them for 24 hours. The hospital should have provided a container. I suspect the second sample was disposed of in the sluice and, I wouldn’t have minded if they’d told me, because I’m a stickler for getting results. I also took the second sample into my GP’s where I have my blood tests taken and I later read that the sample must be kept cold which I doubt is not possible for such a large container. That’s another reason why urine samples aren’t regularly used and it’s more expensive too.
You can’t have too many tests but the urine test, I suspect, is just to make quite sure they aren’t missing something and they do it once at the outset along with the blood test. If they’re looking for myeloma then usually they perform a bone marrow biopsy and a skeletal survey.
One single test on its own is not usually definitive one way or another, though it gives you a starting point.