What is the CT density of uric acid stones?
Typically 100-200HU (compared to >400 for most calcium containing stones).
What proportion of all stones are uric acid stones? What are the other ones?
Uric acid only account for 5 - 8% of all stones. Calcium oxalate + / - calcium phosphate (75%), struvite (triple phosphate) (15%) and pure calcium phosphate (5 - 7%) make up the rest, with cystine (1%) and AZT stones being rare.
What can you surmise about the composition of this calculus?
The patient has gout and this sizeable rectangular stone was not visible on a plain film KUB. As such it most likely a uric acid stone, which are radiolucent on KUB films, but can be easily detected on CT, for they are denser than soft tissues or blood clots. The also tend to have a flat, sheet-like form.
Oblong calculus is located in the right renal pelvis (blue dotted line) with stranding of the proximal ureter (yellow arrow).