Elsevier

Urology

Volume 83, Issue 1, January 2014, Pages 45-49
Urology

Endourology and Stones
External Validation of a Preoperative Renal Stone Grading System: Reproducibility and Inter-rater Concordance of the Guy's Stone Score Using Preoperative Computed Tomography and Rigorous Postoperative Stone-free Criteria

https://doi.org/10.1016/j.urology.2013.09.008Get rights and content

Objective

To validate the Guy's stone score (GSS) using preoperative computed tomography (CT) and to assess its inter-rater concordance and association with rigorous definitions of stone clearance.

Methods

The preoperative CT scans of 166 consecutive percutaneous nephrolithotomy (PCNL) patients treated by a single surgeon were independently reviewed by 2 urology residents and graded according to GSS. Concordance was calculated using Cohen's kappa score. Residual fragments (RFs) were evaluated on CT or plain radiography on postoperative day 1. GSS was correlated with 3 different outcomes; RFs <4 mm, RFs <2 mm, or no RFs.

Results

Higher GSS was associated with decreased stone clearance by any metric on a CT scan: RFs <4 mm (P = .03), RFs <2 mm (P = .02), or no RFs (P = .02). On plain radiography, higher GSS was only associated with lower likelihood of no RFs (P <.005). Inter-rater concordance was good (κ = 0.72), with 78% of cases categorized the same by both raters. Twelve of 36 cases (33%) of disagreement were between categories II and III and 20 of 36 cases (56%) pertained to unclear definitions of “partial staghorn stone” and “abnormal anatomy.”

Conclusion

The GSS is a straightforward grading system of the complexity of renal stones. When applied to preoperative CT scans, it offers good inter-rater concordance and is associated with rigorous endpoints of stone clearance. The inter-rater concordance could be further improved by explicit definitions of abnormal anatomy, partial vs complete staghorn stones, and the size of a calculus that constitutes a separate stone.

Section snippets

Material and Methods

In accordance with the approval of our institutional review board, the medical charts of 166 consecutive patients who underwent PCNL by a single fellowship trained endourologist at our institution between July 2008 and February 2013 were retrospectively reviewed. Inclusion criteria included stones located in the kidney (solitary ureteral stones were excluded) identified on preoperative CT imaging, and the presence of postoperative imaging (low-dose CT or abdominal x-ray) on postoperative day 1.

Results

GSS inter-rater concordance was good, with 130 of 166 cases (78%) categorized the same by both raters (κ = 0.72, 95% confidence interval 0.61-0.80). Although the rating only differed by 1 grade in 25 cases (15%), in 11 cases (7%) there was a 2 category difference between raters.

As shown in Table 2 the most common disagreement (10/36, 27%) occurred between grades I and II. Most cases of disagreement (20/36, 56%) resulted from the absence of clear definitions for “abnormal anatomy” as well as

Comment

The current study validates the GSS as a simple and reliable tool for predicting stone clearance, using preoperative CT evaluation and rigorous endpoints (0 mm) for stone clearance regardless of imaging modality. The GSS was also validated for CT using stone free cutoffs of <2 mm and <4 mm, but this did not hold true using abdominal x-ray, which is a less sensitive examination for small fragments. The simple design of the GSS makes it potentially useful for counseling patients regarding the

Conclusion

The GSS is a straightforward grading system of the complexity of renal stones. When applied to preoperative CT scans, it offers good inter-rater concordance and is associated with rigorous end points of stone clearance. The inter-rater concordance could be further improved by explicit definitions of abnormal anatomy partial vs complete staghorn stones, and the size of a calculus that constitutes a separate stone.

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    The significant positive correlation of complication with increasing the grade of GSS is still a matter of debate. Thomas et al. [7] who described the GSS failed to find any positive correlation between either overall rate or severity of complications and GSS, Ingimarsson et al. [17] and Noureldin et al. [19] also in their studies did not find any significant correlation with complications. In contrast Mandal et al. [10] in their big study on 221 renal units that mainly evaluated this issue, found a significant relation between GSS and rate of complication, this positive correlation has also been proved in the studies of Sinha et al. [16] and Vicentini et al. [3].

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Financial Disclosure: The authors declare that they have no relevant financial interests.

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