Outer diameter of the vermiform appendix as a sign of acute appendicitis: Evaluation at US

PURPOSE: To evaluate the usefulness and limitations of the outer diameter o f the vermiform appendix at cross-sectional ultrasonography to confirm or r ule out acute appendicitis. MATERIALS AND METHODS: In a prospective study, outer appendiceal diameters in 240 control subjects and in 278 patients suspected of having acute appen dicitis who did (n = 98) or did not (n = 180) have acute appendicitis were measured. RESULTS: Outer appendiceal diameters in the control subjects ranged between 2 and 13 mm, and in 55 (23%) of 240 control subjects, diameters were 6 mm or more. Diameters in the symptomatic patients without acute appendicitis r anged between 2 and 11 mm, and 57 (32%) of 180 patients had diameters of 6 mm or more. Diameters of acutely inflamed appendices ranged between 6 and 3 0 mm. A diameter of 6 mm or more confirmed acute appendicitis with a sensit ivity of 100%; a specificity of 68%; positive and negative predictive value s of 63% and 100%, respectively; and an accuracy of 79%. CONCLUSION: The outer appendiceal diameter of 6 mm or more as a sign of acu te appendicitis provides high sensitivity but limited specificity. This dia gnostic criterion is more useful in excluding acute appendicitis than in co nfirming it.