To examine the significance of the carbonation rate (CR) in carbonated apatite (carbapatite [CA]) stones and its relationships with the morphologic characteristics of CA and etiology. CA stones without struvite can result from metabolic disorders or urinary tract infection, but the latter etiology is still debated. Infection stones caused by urea-splitting bacteria are made of CA admixed with struvite and exhibit a high CO₃²⁻/PO₄³⁻ ratio (CR). However, little is known as to the significance of the CR of CA in the absence of struvite in idiopathic calcium phosphate stones.

We studied 39 urinary calculi mainly composed of CA without struvite. Of the 39 patients, 13 had a past or present history of urinary tract infection, 24 had hypercalciuria, and 2 had medullary sponge kidney. The stones were examined by Fourier transform infrared spectroscopy and scanning electron microscopy. The presence of amorphous carbonated calcium phosphate or whitlockite was also considered.

The CR of CA was 14% ± 9%. On scanning electron microscopy, the CA particles appeared as spherules of 4.5 ± 3.0 μm in diameter and were significantly larger in females than in males. In 16 cases, scanning electron microscopy showed bacterial imprints. In these calculi, the CR was significantly greater (22% ± 7%) than in those without a visible bacterial imprint (8% ± 5%, P < .0001). Amorphous carbonated calcium phosphate was found in 15 of 16 stones (93.8%) with imprints and in none of the 23 stones without imprints (P < .0001).

A close relationship was observed between the presence of bacterial imprints, indicative of past or current urinary tract infection, and both the presence of amorphous carbonated calcium phosphate (or whitlockite) and a high CR of CA.