Abstract
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In the PCNL setting, several preoperative scoring systems have been introduced for systematic and quantitative assessment of kidney stone complexity, such as the Guy's stone score, 1 the Clinical Research Office of the Endourological Society nephrolithometry, 2 and the S.T.O.N.E. (stone size [S], tract length [T], obstruction [O], number of involved calices [N], and essence or stone density [E]) nephrolithometry nomogram. 3 In addition to imaging characteristics and surgeon experience, patient features that may contribute to surgical outcomes are considered in these systems, including obesity, renal surgical history, spinal cord injury, and spina bifida status, noting that not all of these parameters are used uniformly in all systems.
Validation is one of the first steps in the standardization process of a scoring system. External validation aims to address the accuracy of a model in patients from a different but plausibly related population, which may be defined as a selected study population representing the underlying disease domain. Sfoungaristos and associates used the Guy's stone score to predict postoperative outcomes and complications and to externally validate it as an independent predictor for stone-free status. The reported results in this study fall into the accepted range of stone-free rates and perioperative complications, further strengthened by the fact that they homogenized imaging (only CT scans used preoperatively and postoperatively), which the original scoring system did not. As a negative, acknowledged by the authors, a single surgeon (with a lower pole access preference) performed all of the procedures, eliminating interobserver reliability.
A recent publication aimed to compare each scoring system in the same cohort, with the goal of elucidating which was the most predictive of surgical outcomes. Although all scoring systems do not share the same variables, all three were comparable in predicting stone-free rates. Regarding parameters such as estimated blood loss or hospital stay, the Guy's score and S.T.O.N.E. nephrolithometry showed the best prediction accuracy. The authors concluded that a single scoring system should be adopted to unify reporting. 4
Surgical candidates for stone disease are a widely divergent population. Body habitus, preexisting comorbidities, and surgeon experience are all factors amid many that may have an impact on surgical outcomes. Of these, stone size (as consistently and reliably dictated by previous reports and literature) remains indisputably the most prominent predictor of PCNL operative outcomes.
In the future, perhaps, an unified scoring system for patients with nephrolithiasis, not only undergoing PCNL but any type of stone related surgery, will be available, thus achieving optimal treatment planning for patients whose characteristics may predict poor outcomes and, if possible, avoid them.