STONE Score Close, But Not Quite

The STONE score is a lovely idea for a common problem – ureteral stones are a frequent source of low-value imaging in the Emergency Department.  An adequately sensitive or specific decision-instrument would improve diagnostic accuracy while reducing imaging.  Unicorns would bloom from the frozen tundra.

STONE, however, has a couple issues.  Firstly, it was derived and validated at a single institution in similar cohorts.  Secondly, it has a few variables that probably lack face validity, i.e., three points for non-black ethnicity?

This report is from the large, multi-center trial comparing CT vs. ultrasound for the diagnosis of ureterolithiasis.  Of the 2,759 patients randomized from the original trial, 845 underwent CT as gold standard and had the data available for evaluation of the STONE score.  This constituted their retrospective validation cohort.

So, STONE is OK.  Not great, but OK.  The AUC of STONE was 0.78, which is a step up from 0.68 of physician gestalt.  However, it was only 87% specific at the high end in the validation to rule-in stone, and, then, 96% sensitive at the low end as stone rule-out.  Phrased otherwise, there were about 10 to 20% fewer cases of ureterolithiasis identified in the moderate and high cohorts in this validation, compared with the original.  There were also, unfortunately, a greater number of significant alternative diagnoses in the low and moderate cohorts, as well.

The authors come to very reasonable conclusions.  First, the STONE score isn’t perfect.  It can be a useful adjunct to risk-stratification and shared decision-making, but the positive and negative likelihood ratios are inadequate as a standalone rule.  As a corollary, the score definitely requires refinement or reinvention, based on their analysis of the predictive contribution of different variables in the original score.  It is probably still fair to use this score to supplement gestalt in the context of pursuing judicious use of resources, but it must be considered in the context of other predictive features to determine the appropriate imaging strategy.

“External Validation of the STONE Score, a Clinical Prediction Rule for Ureteral Stone: An Observational Multi-institutional Study”
http://www.ncbi.nlm.nih.gov/pubmed/26440490