PPi is present in plasma at a concentration of 1–6 μM  and in urine levels are around 10 μM . Calcium phosphate stone formers appear to have reduced urinary PPi excretion when compared with control subjects [20–23]. Intravenous 32PPi is rapidly hydrolyzed in plasma by tissue nonspecific alkaline phosphatase (Tnsalp) that is expressed in the proximal tubules of the kidneys  and less than 5% of intravenous 32PPi appears in urine. These data indicate that urine PPi is generated locally in the kidneys [25, 26].