A comparison between 24h urine collection and overnight spot urines in evaluating the risk of stone disease

: Despite being recommended by most guidelines, the metabolic evaluation of patients with nephrolithiasis has limited diffusion due to difficulties relating both to the access to laboratory investigations and to urine collection modalities. Consequently, in addition to the classical 24-h collection, alternative and simplified collection modes have been proposed. We report here on the comparison between metabolic evaluation carried out on 24-h double collection (Lithotest) and overnight spot urines (RF test). Fifty-four patients with stone disease were enrolled, excluding patients with infection or cystine stones. For Lithotest, we measured all analytes necessary to calculate state of saturation (ß) with calcium oxalate, brushite and uric acid, by means of Lithorisk.com. For RF, we measured calcium, magnesium, oxalate, citrate, sulphate, phosphate, pH and creatinine. The comparison was made with creatinine ratios. An estimate of ßCaOx, ßbrushite and ßAU was obtained also on RF urines by using simplified algorithms. We found highly significant correlations between all parameters, despite quite different means. There was a nice correspondence between the two sets of measurements, assessed by the Bland-Altmann test, for calcium, oxalate, citrate, sulphate, urate and pH. Overnight urine had higher saturations compared to 24-h one owing to higher concentration of the former. In conclusion, RF test on overnight urine cannot completely replace Lithotest on 24-hr urine. However, it can represent a simplified tool for either preliminary evaluation or follow-up of patients with stone disease.