Computed tomography measurement of tophus volume: Comparison with physical measurement

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Abstract
Objective: Computed tomography (CT) has high accuracy for tophus detection. This study assessed reliability of CT measurement of tophus volume and compared reproducibility of CT with physical measurement of tophus size.Methods: Forty‐seven hand tophi were analyzed in 20 patients with gout. The longest tophus diameter was recorded by 2 independent observers. All patients proceeded to CT scanning of the hands on a Philips Brilliance scanner (0.8‐mm slices). Two independent observers measured tophus volume using the Surface Shaded Display 3‐dimensional function on the Philips CT workstation. Five patients underwent repeat physical and CT assessments within 1 week (18 observations). Inter‐ and intraobserver reproducibility were analyzed by limits of agreement and coefficients of variation.Results: Of the 47 lesions identified as tophi on physical examination, 42 (89%) were also identified on CT. The mean (95% confidence interval [95% CI]) difference between observers for physical measurement was 0.45 mm (−4.07, 4.96) and for CT was 65.2 mm3 (−293.0, 423.3). The mean (95% CI) difference between visits for physical measurement was −0.72 mm (−5.47, 4.03) and for CT was −13.1 mm3 (−112.5, 86.3). There was no difference between coefficients of variation for inter‐ and intraobserver reproducibility for the 2 measurement techniques. For tophi identified by physical and CT assessment, there was good correlation between measurements (r = 0.91, P < 0.0001).Conclusion: CT assessment of tophus volume is reliable and reproducible. However, physical measurement correlates well with CT and has equivalent reproducibility. These data support the use of physical measurement as a simple and reliable method to assess tophus size.