Novel Diagnostic Test Assesses Risk of Urinary Uric Acid Crystallization

Uric acid (UA) kidney stones account for 10% to 11% of all kidney stones, and this percentage has increased over time. An accurate, rapid, simple, and low-cost test is needed to distinguish urine that is susceptible and resistant to the formation of UA crystals.

Renal stone formation is a multifactorial process that is affected by urine composition and other factors, such as structural and pathological features of the kidneys, which include the presence of renal cavities with low urodynamic efficiency that retain urine for long periods, and alterations of the epithelial cells that cover the renal papillae.


Urology Scientists at the University of Balearic Islands (Palma de Mallorca, Spain) and their colleagues collected urine samples from 20 healthy adult volunteers and 54 active formers of UA stones. Three samples were collected from each participant, with at least seven days between each collection. The main lithogenic parameters for UA stones were determined, and the risk of UA crystallization (RUAC test) was performed in all urine samples.

The team reported that the ROC analysis indicated the area under the curve (AUC) was 0.881 (95% CI: 0.831 to 0.932), the sensitivity was 78.5%, the specificity was 92.4%, and the Youden index was 0.709. Based on this analysis, the optimum cutoff value was five positive wells. The diagnostic specifications of RUAC test data, when considering that a urine is lithogenic with respect to UA when its SS is greater than 2 and the RUAC test is positive when crystals formed in five or more wells. A total of 61% of basal urine from UA stone formers were positive in the RUAC test, but only 19% of healthy volunteers were positive in the basal urine. Furthermore, among UA stone formers, the percentage of patients with a positive RUAC test decreased from 61% to 40% after treatment with theobromine. However, among healthy volunteers, the decrease after treatment with chocolate powder was not significant.

The authors concluded the RUAC test has high diagnostic accuracy and low cost, and is also rapid and simple. This test can therefore be used as a screening tool in clinics and reference laboratories, in locations with limited laboratory infrastructure, and by community health workers with minimal training. This test demonstrates that the value of uric acid supersaturation in a urine, would also be a useful parameter, since for SS>2, the urine would be lithogenic for uric acid. The study was published on May 6, 2021 in the journal Clinica Chimica Acta.

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