Urinary stone size: comparison of abdominal plain radiography and noncontrast CT measurements.

BACKGROUND AND PURPOSE: To compare urinary stone size as measured by abdominal plain radiography (AXR) with stone size as measured by noncontrast three-dimensional spiral CT in patients with acute renal colic.

PATIENTS AND METHODS: Patients presenting to the emergency room of a single institution with urinary stones that were visible on both AXR and noncontrast spiral CT were identified. Two radiologists blinded to the clinical outcomes separately and randomly reviewed all films and measured maximum longitudinal (craniocaudal) and transverse (anteroposterior) stone diameters. The two-tailed paired Student's t-test was used to compare the sizes of each stone on AXR and CT.

RESULTS: Over a 1-year period, 22 patients were identified with a total of 31 urinary stones visible on both AXR and CT. Nineteen stones were located in the kidney, three in the midureter, and nine in the distal ureter. The mean stone size by AXR was 6.1 mm (range 2-13 mm; SD +/- 1.95) in the longitudinal axis and 5.3 mm (range 2-11 mm; SD +/- 1.50) in the transverse axis. The mean stone size by CT was 6.9 mm (range 3-12 mm; SD +/- 1.95) in the longitudinal axis and 6.1 mm (range 2-11 mm; SD +/- 1.50) in the transverse. The differences between AXR and CT measurements did not attain significance in either the longitudinal (p = 0.67) or the transverse (p = 0.25) axis.

CONCLUSIONS: A CT scan provides estimates of stone size that are consistently greater than those of AXR in both the longitudinal and transverse axes. However, for stones between 2 and 13 mm in maximum diameter, these differences do not attain significance. In patients with a history of radiopaque stones in this size range, therefore, AXR may provide useful size data for clinical decision-making without concern about significant disparities between the two modalities. As AXRs are more expeditiously obtained, incur less direct costs, and expose patients to significantly lower doses of radiation than CT scans, they remain a useful adjunctive study in the work-up of nephrolithiasis.