The MIMIC score was designed to determine the likelihood of spontaneous ureteral stone passage (SSP) in patients presenting with renal colic and managed conservatively. The MIMIC score is widely used in clinical practice to counsel patients regarding their likelihood of successful SSP, however its use in cohorts outside of the United Kingdom has not been validated. The aim of this study was to determine the external validity of the MIMIC score.

This was retrospective cross sectional cohort study that reviewed patients who presented to the emergency department (ED) of a tertiary referral centre with renal colic and imaging proven ureteral calculus between 2021 and 2022. All factors required for calculation of the MIMIC score were recorded, in addition to patient demographic details. The outcome of SSP was determined by review all private and public imaging providers within our state, as well as review of any ED or inpatient admissions to hospitals in our state. The calculated score was compared to the observed outcome. Discrimination was determined using an area under the receiver-operating curve with an AUC >0.7 considered validated and calibration using calibration plot.

397 patients were included. The median age was 49 (IQR 37–60). 79% were males. The mean stone size for successful and non-successful SSP was 3.8 mm and 4.8 mm respectively. 88% of stones passed spontaneously. The AUC was 0.68 (95% CI: 0.60–0.77). The calibration plot showed an underestimation of SSP. In the lowest quintile of patients with a mean predicted SSP rate of 46%, the observed SSP rate was 74% (95% CI: 63–83%). With a higher predicted SSP rate, the MIMIC score was accurate. In the highest quintile of patients with a mean predicted SSP rate of 90%, the observed SSP rate was 92% (95% CI: 84–97%).

In this population, the prediction of the MIMIC score were not sufficiently accurate. There was a tendency to underestimate the likelihood of SSP in our cohort, particularly in patients whose MIMIC score estimated a low likelihood of passage. The MIMIC score was accurate for patients who had a higher likelihood of SSP (>83%). Clinicians outside the UK should use the MIMIC score with caution, further scoring systems are required outside of this population.