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External validation of Guy’s stone score, reproducibility and inter-rater concordance using pre-operative computed tomography and strict stone free criteria
Johann P. Ingimarsson, MD, Lawrence M. Dagrosa, MD, Elias S. Hyams, MD, Vernon M. Pais, MD.
Dartmouth-Hitchcock Medical Center, Lebanon, NH, USA.
BACKGROUND: Guy’s stone score is a novel simple grading system developed to assess renal stones complexity pre-operatively in patients undergoing percutaneus nephrolithotomy (PCNL). In the original paper describing the system, stones were assessed on KUB and stone free rates were defined as no stones under 4mm. The authors reported high inter-rater concordance and good correlation with stone free state. CT scans are commonly used for pre-operative evaluation and many authors argue for stricter stone free state definitions. If validated on CT scans using strict stone free criteria, Guy’s stone score could serve as a simple and fast tool to advice patient on the likely of needing a staged procedure to achieve a stone free state.
METHODS: The pre-operative CT scans of 103 consecutive PCNL patients operated by a single surgeon, were independently reviewed by two urology resident reviewers and graded according to Guy’s stone score as reported by Thomas et al. Inter-rater concordance was assessed with Cohen’s kappa. Residual stones were evaluated on CT or abdominal x-ray on post-operative day 1. Three different stone free definitions compared; defined as a) no fragment > 4 mm, b) no fragment < 2 mm (p<0.001) or no fragment identified on imaging (p<0.001)
Correlation was calculated according to three definitions of a stone free state
RESULTS: The inter-rater concordance was good, with 78 of 103 cases categorized the same by both raters (Ƙ=0.66). Increasing Guy score correlated with stone free rates, as defined by calcification < 4 mm (p<0.01), < 2 mm (p<0.001) or no calcification identified on imaging (p<0.001). The most common disagreement, 12 of 25, was between categories II and III. A large part of the disagreement (15/25) resulted from absence of clear definitions of abnormal anatomy, partial stag horn stone and stag horn stone.
CONCLUSIONS: Guy’s stone score is a simple reproducible scoring system that accurately predicted the stone-free rates after PCNL. It is valid for CT scans and various definitions of stone free states. Inter-rater correlation may be further improved by adding definitions of abnormal anatomy, partial stag horn stone and stag horn stone.
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