The Predictive Accuracy of Hounsfield Units and Urinary pH in the Non-invasive Diagnosis of Radiolucent Urinary Stones

Introduction Radiolucent urinary calculi, primarily uric acid and struvite stones, pose a diagnostic challenge due to their indistinct appearance on plain radiographs. Differentiating between these two types is essential, as they require markedly different therapeutic strategies. This study evaluates the combined utility of Hounsfield units (HU) from non-contrast computed tomography (NCCT) and urine pH values as non-invasive predictors of stone composition in radiolucent calculi. We hypothesized that the combination of HU and urine pH can accurately differentiate between uric acid and struvite stones. Methods We conducted a retrospective cohort study of 110 adult patients with radiolucent stones confirmed by chemical analysis between January 2021 and December 2024 at Queen Elizabeth Hospital, Birmingham. HU values were measured on NCCT, and urine pH was calculated as the mean from all available urinalysis records over the preceding six months. Infection markers (nitrite and leukocyte esterase) were also recorded. All patients underwent a X-ray kidneys, ureters, and bladder (KUB) to confirm radiolucency. Patients with mixed or radiopaque stones were excluded. Results Of the 110 patients, 65 had uric acid stones and 45 had struvite stones. Uric acid stones exhibited significantly lower HU values (432±98) and more acidic urine (pH: 5.5±0.3) compared to struvite stones (HU: 694±182, pH: 6.7±0.4; p<0.001 for both). Nitrite and leukocyte esterase positivity were also significantly higher in the struvite group (82% and 84%, respectively) than in the uric acid group (24% and 38%; p<0.001). A diagnostic threshold of HU ≤500 and pH ≤5.5 predicted uric acid stones with 87% sensitivity and 83% specificity, while HU >600 and pH >7 predicted struvite stones with 85% accuracy. Receiver operating characteristic (ROC) curve analysis demonstrated an area under the curve (AUC) of 0.94, indicating excellent diagnostic performance. Conclusion HU and urine pH are effective, non-invasive tools for differentiating uric acid from struvite radiolucent stones. Their combined use enables early, targeted medical management and may reduce reliance on delayed surgical confirmation, especially in systems with long elective wait times. Incorporating these parameters into routine clinical triage could significantly improve outcomes in patients with radiolucent urolithiasis.