In gout, do MSU crystals form only in tissue, or are there circulating microcrystals of MSU within blood vessels?   

The solubility of MSU is dependent upon solute, temperature, and pH. Even with a rapid rise in total body uric acid as seen with tumor lysis syndrome, where uric acid levels can exceed 15 mg/dL, I have never heard of an MSU crystal identified in a serum sample. As I explain to students and patients...

This is a great question and I am not sure that we have a definitive answer. We do know that unlike CPP crystals which require a solid matrix in which to form, MSU crystals can form in solution, but whether this occurs in the circulation is unclear. One could imagine that with very high levels of ci...