Contemporary accuracy of digital abdominal X-ray for follow-up of pure calcium urolithiasis; is there still a role?

Abstract

Radiological imaging remains the cornerstone of follow-up of patients with urolithiasis. Plain abdominal X-ray (XRKUB) of these patients is largely becoming obsolete in favour of non-contrast computed tomography of the abdomen and pelvis (CTKUB), which offers superior sensitivity and specificity. Contentiousness exists however, related to the radiation doses associated with CT. Calcium oxalate remains the most common stone composition across most populations. These calculi are radiopaque and thus follow-up with XRKUB may be appropriate in some settings, avoiding the higher radiation doses associated with standard protocol CT. With the emergence of low dose CT however, and the modest accuracy of XRKUB, the ongoing role for XRKUB in the management of urolithiasis is debatable. In this study we assessed the proportion of pure calcium urolithiasis visible on XRKUB in order to assess the utility of XRKUB for follow-up of pure calcium urolithiasis. Hospital laboratory databases were analysed to identify patients who had undergone urological intervention and extraction of urolithiasis composed of pure calcium salts, who had undergone CTKUB and XRKUB during the same episode for diagnosis of the calculus. 105 calculi were included and the imaging for each patient analysed. 79/105 calculi (75%) identified on CT were visible on XRKUB. The median calculus visible on XRKUB was 7.0 mm (IQR: 5.0 - 10.0mm), the median calculus not seen was 4.25mm (IQR: 3.5 - 7.0mm) as measured on CT (p<0.01). XRKUB accuracy varied according to calculus position within the renal tract. Calculi were most often visible in the renal pelvis and proximal ureter (86%), 50% of mid-ureteric and 72% of distal ureteric calculi were visible (p=0.01). Until low dose CT protocols become widely adopted, XRKUB still has a limited role in the management of pure calcium urolithiasis in selected patients. Due to its modest accuracy in many settings this role is limited mainly to larger proximal urolithiasis.