Who Cares About Stone Analysis?

Almost universally, my patients want to know what their stone analysis shows after endoscopic stone surgery. Many call it a “biopsy,” with the impression that the stone composition provides all necessary information regarding what caused the stone and what can be done to prevent another stone. The prospect of another symptomatic stone event—and even worse, a stent—motivates many patients to develop a game plan for stone prevention. The AUA and European Association of Urology guidelines recommend stone analysis as part of medical prevention when a stone is available, either from spontaneous passage or surgical removal.^1,2 Endoscopic stone surgery provides the opportunity to remove symptomatic stones and also to capture stones for determination of their chemical composition. Laser fragmentation with basket extraction of stones provides an easy mechanism to obtain stones for analysis. However, with laser stone dusting technique, the surgeon does not necessarily need to extract stones at the time of surgery for chemical analysis.

Similar to scenarios in which a patient may have passed a stone without being able to capture it for chemical analysis, urologists may have to use additional clues that could suggest a particular stone type. Hounsfield units on CT scans, radiolucency or opacity on plain-film x-ray, family history of cystine stones, microscopic urine crystal analysis, and prior stone analysis data are commonly utilized tools. These characteristics have for many years assisted urologists in identifying stones with compositions, especially for more unique stones’ compositions such as uric acid, cystine, struvite, and other rare stones. Recent studies have evaluated alternative ways to obtain stone composition during endoscopic stone surgery that would be compatible with a pure dusting technique. In this article we will see the pros and cons of 2 of these techniques.

Collecting Stone Dust Intraoperatively for Chemical Analysis

Sierra and colleagues³ recently evaluated the concordance in stone composition between stone fragments collected using basket extraction and the irrigation fluid containing stone dust aspirated from the same 20 patients from ureteroscopy with thulium fiber laser lithotripsy. All samples were taken intraoperatively from the same patients and analyzed using infrared spectroscopy, and all had dust samples sufficient for analysis. The authors observed that 25% had 1 stone component missing between either the aspirated dust or the fragment, all of whom had multiple stone types present in the samples. All 3 patients with cystinuria had cystine present in the stone analysis from both aspirated dust and basketed fragment samples. Additionally, patients with a uric acid stone did have this stone type detected in both dust and basket specimens. Previously, a similar study was performed in 97 patients undergoing ureteroscopy with holmium laser lithotripsy.⁴ The starkest contrast between this study and the prior was that only 68% of patients had dust sufficient for analysis. However, these authors observed that among patients with dust sufficient for analysis, 26% of these patients had at least 1 stone type absent between either the aspirated dust or the fragment; of these 26%, all had mixed stone compositions.

Endoscopic Stone Recognition

One potential tool to assess stone composition is experience-based visual stone identification during endoscopic stone surgery, also called endoscopic stone recognition (ESR). This technique relies on a urologist’s ability to accurately predict stone composition from its appearance during endoscopic surgery alone. The results for this method have not been overly promising. Among 15 endourologists reviewing 100 videos recorded with high-quality digital ureteroscopes, Henderickx et al⁵ found wide-ranging diagnostic accuracy among stone types. For pure stone types, urologists recognized calcium phosphate stones 13% of the time, uric acid 22%, magnesium ammonium phosphate 40%, calcium oxalate dihydrate 50%, calcium oxalate monohydrate 59%, and cystine 80%. The authors rightly concluded that the diagnostic accuracy of ESR is “limited” based on the results of this study.

Randall and colleagues⁶ found a similar percentage of diagnostic accuracy. Among endourologists polled, the overall accuracy was 44% for all stone types. Compared to the Henderickx study, this group interestingly found a much lower accuracy of cystine stones at only 14.2%, but a similar 65.9% accuracy for calcium oxalate monohydrate stones. Taken together, these 2 studies suggest that ESR is not a particularly accurate tool for most urologists.

Where Do We Progress From Here?

Our guidelines still suggest stone analysis as an integral part of our longitudinal management in patients with nephrolithiasis. Even if using empiric medical management, this strategy generally relies on knowledge of a patient’s stone type.^7,8 Patients with a known stone composition may be the best candidates for missing a stone analysis if electing a dusting technique unless current stone prevention strategies have failed and assessment for a change in a patient’s stone composition could aid in management. Intraoperative dust collection during endoscopic stone surgery with dusting technique does appear to provide a feasible and efficacious method of obtaining these data, giving urologists the flexibility to use this technique while still getting our stone biopsy data. Endoscopic stone recognition, on the other hand, does not seem to be a reliable option at this point in time. In the ever-evolving world of artificial intelligence, we may have the possibility to identify stone composition or stone prevention strategy without traditional chemical analysis in the future.