Radiographs Reveal Key Clues to Predicting Canine Bladder Stone Type

Journal of Veterinary Internal Medicine, 2025

Author

Obi Veterinary Education
November 10, 2025

Diego Pulido Vega, Mathieu Motteau-Lévêque, Christelle Maurey, Jeremy Mortier

Background

Urolithiasis is a common condition in dogs, and identifying the urolith mineral composition (UMC) is essential for guiding treatment and prevention strategies. While calcium oxalate and struvite stones are most common, cystine stones are less frequent. Traditionally, mineral composition prediction relies on ex vivo analysis or subjective assessment. This study sought to describe in vivo radiographic features associated with different UMCs and to identify objective radiographic characteristics that can aid non-invasive differentiation among calcium oxalate, struvite, and cystine uroliths.

Methods

A retrospective cross-sectional study reviewed medical records of dogs with confirmed lower urinary tract (LUT) uroliths examined at the École Nationale Vétérinaire d'Alfort between 2009 and 2023. Dogs were included if their uroliths contained ≥70% of a single mineral (calcium oxalate, struvite, or cystine). Lateral abdominal radiographs taken within one week before urolith removal were analyzed for urolith size, shape, surface, borders, and internal architecture. Quantitative opacity was determined using a urolith-to-L5 mean pixel value ratio (UOR). Statistical comparisons among UMC groups were performed using ROC curve analysis and ANCOVA.

Results

A total of 202 dogs met inclusion criteria (109 calcium oxalate, 58 struvite, 35 cystine). Calcium oxalate uroliths were the most radiopaque and frequently associated with concurrent nephroliths and distinctive shapes (bosselated, spiculated, or cylindrical). Struvite uroliths were significantly larger (median 15 mm; IQR 8.1–25.8) and more likely solitary, with an optimal size threshold of ≥11 mm for predicting struvite (AUC = 0.82; specificity 91%, sensitivity 67%). Cystine uroliths were the least radiopaque (median UOR 0.71; IQR 0.63–0.80), with a diagnostic cut-off of UOR ≤ 0.83 for identifying cystine (AUC = 0.81; specificity 67%, sensitivity 84%). Struvite stones were associated with positive bacterial cultures (67%, mainly Staphylococcus spp.), whereas calcium oxalate and cystine stones were typically sterile.

Limitations

Being retrospective, the study was limited by variability in radiographic acquisition parameters, prior dietary or medical interventions, and potential underestimation of small or radiolucent stones. The analysis was restricted to three UMC types, excluding urate and silica due to limited sample sizes. Differences in radiograph technique and body composition among dogs may have influenced UOR measurements.

Conclusions

In vivo radiographic assessment provides valuable, non-invasive insights into the mineral composition of canine LUT uroliths. Larger, moderately radiopaque, pyramidal stones are likely struvite; small, highly radiopaque, bosselated or spiculated stones suggest calcium oxalate; and mildly radiopaque stones with low UOR values indicate cystine. Quantitative measures such as urolith size and radiopacity ratios enhance diagnostic confidence and may help guide medical versus surgical management decisions.

Measurements of the ratio (labeled UOR) of the mean pixel value (MPV) of the largest cystolith to the L5 MPV on lateral abdominal radiographs in three dogs. A circular ROI is positioned at the caudal third of L5 vertebra (black circle) with care to avoid superimposition with cortical bone and vertebral transverse processes, and a circular ROI is placed on the largest cystolith (white circle) with care to incorporate as much of the urolith as possible without including its peripheral margins and to avoid superimposition with other uroliths, digestive structures and potential bubbles of gas due to prior catheterization. (A) Bosselated calcium oxalate cystolith in a dog. The MPV of urolith is 2015; the MPV of L5 is 1952; the UOR value is 1.03. (B) Large pyramidal struvite cystoliths and one similar urethrolith in a dog. The MPV of urolith is 1542; The MPV of L5 is 1875; The UOR value is 0.82. (C) Multiple small faceted cystine cystoliths in a dog. The MPV of urolith is 1253; The MPV of L5 is 1914; The UOR value is 0.65.

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