Ureteral stone passage accurately predicted by model

July 1, 2010

A model predicting the time for stone passage using radiographic parameters was recently validated and now appears to be ready for clinical use.

Barcelona, Spain-For urologists caring for patients with ureteral stones, uncertainty about how long it will take for a stone to pass spontaneously is a common concern. A model predicting the time for stone passage using radiographic parameters was recently validated and now appears to be ready for clinical use, researchers reported at the European Association of Urology annual congress in Barcelona, Spain.

At the meeting, researchers from the General Hospital of Veria, Greece, presented data validating a previously developed prediction model.

"This formula works to educate patients about stone colic and the length of time they can expect it," said study co-author Dimitris Kikidakis, MD, attending urologist at the General Hospital of Veria, who worked on the study with Petros Sountoulides, MD, PhD, and colleagues.

In the current study, Dr. Sountoulides and colleagues examined 68 patients with acute renal colic and ureteral stones smaller than 6 mm that were allowed to pass spontaneously. The formula used in the calculation was: "Days to passage=20 + 5× size of stone in mm – 4× position," where position=1 for proximal ureter, 2 for mid-ureter, and 3 for distal ureter. The initial formula proposed by Drs. Miller and Kane included a factor for laterality (right vs. left), but this was left out by Dr. Sountoulides' team.

Of the patients included in the study, 94% passed their stones spontaneously within 6 weeks. The model succeeded in accurately predicting the time to passage. When evaluating the size of the stone, average passage times ranged from 12 days for stones smaller than 2 mm to 37 days for those between 4 mm and 6 mm. Similarly, stones in the distal ureter passed in 21 days, compared to 36 days in the proximal ureter. This was similar to what was predicted on the model, especially for stones smaller than 4 mm.

For stones less than 4 mm, Dr. Sountoulides said the model was "very accurate" in estimating the time to stone passage. However, it tended to overestimate the time of passage in those stones between 4 mm and 6 mm.

The model prediction was within 2 to 4 days of the actual time to passage for the majority of the stones, the researchers reported. They found that the laterality of the stone did not affect outcomes, which led them to remove it from their formula.

One potential weakness of the study, the authors acknowledged, was that none of the patients were prescribed alpha-blockers to facilitate stone passage. Such medications have been shown to facilitate spontaneous stone passage, especially for stones in the distal ureter. Because of this, many American patients with ureteral stones are given such medication. Thus, this formula would likely overestimate the time to passage in patients on alpha-blockers.

However, for patients in whom conservative management of ureteral stones without alpha-blockade is desired, this formula appears to be reliable.