Chemical composition of radio-lucent urinary stones in Sri Lanka

Determining the epidemiological characteristics and clinical and radiological predictors of different types of radiolucent urinary stones is important in the Sri Lankan setting, where routine stone analysis is not freely available.

This was a prospective, multi-centre study involving a sample of patients having radiolucent urinary stones. It was conducted in four urology units in Colombo over one year, starting from August 2019. A urinary stone that was not recognisable in plain X-ray KUB was considered a radiolucent stone. The diagnosis of the stone was made by non-contrast computerised tomography (NCCT KUB). Chemical analysis of the stones was carried out using Fourier-transform infrared spectroscopy (FTIR) technology.

A total of 71 patients were included. The male-to-female ratio was 3:1, and the average age was 48 years. Forty-three (60.5%) stones were uric acid stones, while 26 (36.7%) were calcium oxalate monohydrate stones. There was one calcium oxalate dihydrate stone and one apatite stone (1.4% each). In multivariate analysis, advanced age, unilateral stone location and low radiodensity were associated with uric acid stones. A radiodensity of 548 HU or lower in the NCCT KUB was predictive of a uric acid stone with 96.2% sensitivity and 76.4% specificity.

The majority of radiolucent stones in our study are uric acid stones. The presence of a stone with a radiodensity of 548 HU or lower in the NCCT KUB is predictive of a uric acid stone. Calcium oxalate monohydrate stones constitute a significant proportion of radiolucent stones in Sri Lanka.