Not All Patients Need a CT When the Appendix Is Not Seen on Ultrasound: A Scoping Review

Background/Objective: Recent North American guidelines suggest that CT is indicated for further evaluation where ultrasound (US) is negative, although the negative predictive value (NPV) of ultrasound in adult patients when the appendix is not seen remains unclear. To assess the negative predictive value (NPV) of ultrasound in adult patients when the appendix is not seen. Methods: A scoping review of MEDLINE and EMBASE was performed from inception to 13 May 2025 using PRISMA-ScR guidelines to identify studies evaluating the outcome of adult patients where the appendix is not seen on ultrasound, with preference for studies where there were no secondary signs of acute appendicitis (right lower quadrant free fluid, abscess, ileus, echogenic fat or regional lymphadenopathy). Original studies with at least 10 patients were included in the review. The reference standard included a combination of clinical follow-up, CT and/or pathology. Data synthesis was provided as a qualitative review of the existing literature. Results: Six studies were included in the review. The number of included patients range from 12 to 179 with a mean age of 29-38 years. Few studies reported the patient BMI. NPVs ranged from 80 to 90% for all indeterminate ultrasounds and 83 to 95% for studies where secondary signs of appendicitis were excluded (90 to 95% when non-surgical reference standards were included). Two studies reported NPVs of 96-100% when the pre-test probability was low. Conclusions: The NPV of indeterminate ultrasound for adult patients with right lower quadrant pain and no secondary signs of appendicitis is likely ≥90%. When combined with a low clinical suspicion, the NPV is likely >95%. The appropriateness of a subsequent CT indication when the appendix is not visualized on ultrasound should be determined on an individualized basis.