Do you rely on CT Hounsfield Units to determine stone composition in your patients with recurrent nephrolithiasis who have yet to submit a stone for laboratory based composition testing?  

Of course. I measure it myself if I have the image, and ask radiology to do so if they will. Low values point to UA, for example. It costs nothing and can be informative.

It is somewhat helpful if the value is low, consistent with Uric Acid stones, if higher, not specifically differentiate the calcium stone type. However, since it is part of routine CT stone protocol, it may add some useful information