Background: Contrast-enhanced CT is the reference standard used in diagnostic imaging for acute appendicitis in adults. The radiation dose has been of concern. This study aimed to assess whether a lower radiation dose would affect the diagnostic accuracy of CT.

Methods: This was a prospective single-centre cohort study of patients (aged over 16 years) with suspected appendicitis evaluated for enrolment in concurrent APPAC II-III trials. The diagnostic accuracy of contrast-enhanced low- and standard-dose CT was compared with study protocols guiding imaging based on BMI; this enabled direct CT imaging comparison only in patients with a BMI below 30 kg/m(2). The on-call CT diagnosis was compared with the final clinical diagnosis.

Results: Among all 856 patients investigated, the accuracy of low-dose (454 patients) and standard-dose (402 patients) CT in identifying patients with and without appendicitis was 98.0 and 98.5 per cent respectively. In patients with a BMI under 30 kg/m(2), respective values were 98.2 per cent (434 patients) and 98.6 per cent (210 patients) (P = 1.000). The corresponding accuracy for differentiating between uncomplicated and complicated acute appendicitis was 903 and 87.6 per cent in all patients, and 89.8 and 88.4 per cent respectively among those with a BMI below 30 kg/m(2) (P = 0.663). The median radiation dose in the whole low- and standard-dose CT groups was 3 and 7 mSv respectively. In the group with BMI below 30 kg/m(2), corresponding median doses were 3 and 5 mSv (P < 0.001).

Conclusion: Low- and standard-dose CT were accurate both in identifying appendicitis and in differentiating between uncomplicated and complicated acute appendicitis. Low-dose CT was associated with a significant radiation dose reduction, suggesting that it should be standard clinical practice at least in patients with a BMI below 30 kg/m(2).