Chemical Composition of Radiolucent Urinary Stones: A Fourier Transform Infrared Spectroscopy Study

Abstract Background Radiolucent urinary stones are commonly presumed to represent uric acid calculi; however, the chemical composition of these stones may be heterogeneous. Accurate characterization of stone composition may provide clinically relevant information for understanding stone pathogenesis and optimizing individualized management. This study aimed to determine the FTIR-based chemical composition of radiolucent urinary stones and evaluate its association with demographic and clinical characteristics. Methods This cross-sectional study included 43 patients with radiolucent urinary stones who underwent percutaneous nephrolithotomy or endoscopic lithotripsy at Razi Hospital, Rasht, Iran. Retrieved stones were analyzed using Fourier transform infrared spectroscopy (FTIR). Stone composition was classified based on comparison of overall spectral patterns with reference spectra. Associations between stone composition and demographic characteristics, body mass index, stone size, medical history, dietary pattern, urine culture results, and history of urinary tract infection were assessed. Results Mixed stone compositions were identified in 60.5% of specimens, while pure uric acid stones accounted for 14.0%. The most frequent FTIR-based classification was uric acid plus calcium phosphate (53.5%), followed by pure calcium oxalate (25.6%). Overall, uric acid-containing compositions were identified in 74.4% of specimens. Age was significantly associated with stone composition (p = 0.048), with uric acid plus calcium phosphate becoming more frequent among older patients. No significant associations were observed between stone composition and sex, BMI, stone size, previous medical history, dietary pattern, urine culture results, or history of urinary tract infection. Conclusion Radiolucent urinary stones in this cohort demonstrated substantial compositional heterogeneity and should not be assumed to represent pure uric acid stones based solely on radiographic appearance. FTIR analysis identified frequent uric acid-containing mixed compositions, particularly uric acid plus calcium phosphate. These findings highlight the value of direct stone composition assessment in radiolucent calculi and support further investigation using larger cohorts with detailed metabolic evaluation to clarify the mechanisms underlying mixed stone formation.