From the Guidelines
The normal diameter of the appendix is typically less than 6 mm when measured by CT scan, with a more recent study suggesting an optimal cutoff of 6.6 mm for diameter with compression 1. An appendix measuring greater than 6-7 mm in diameter is considered enlarged and may indicate appendicitis, especially when accompanied by other clinical signs. The appendix is a tubular structure extending from the cecum, averaging 9 cm in length but ranging from 2-20 cm. The diameter measurement should be taken from outer wall to outer wall on cross-sectional imaging. Some key points to consider when evaluating appendix diameter include:
- The frequently referenced cutoff value of 6 mm for maximum outer diameter (MOD) yielded a sensitivity of 97.5%, a specificity of 59.6%, and an accuracy of 73.6% 1.
- Diameter with compression (MOD minus compressible contents) had an area under the curve (AUC) of 0.973 with an optimal cutoff value of 6.6 mm and a sensitivity, specificity, and accuracy of 93.8%, 94.9%, and 94.4%, respectively 1.
- The maximum outer diameter (MOD) had an AUC of 0.967 with an optimal cutoff of 8.2 mm yielding a sensitivity, specificity, and accuracy of 88.8%, 93.4%, and 91.7%, respectively 1. It's essential to note that the appendix diameter can vary slightly based on age, with children having proportionally smaller appendices than adults. When evaluating for possible appendicitis, diameter is just one criterion; wall thickening, periappendiceal fat stranding, appendicolith presence, and non-compressibility on ultrasound are additional important imaging findings. Clinical correlation with symptoms such as right lower quadrant pain, fever, and elevated white blood cell count remains essential for diagnosis, as some individuals may have a slightly larger appendix as their normal baseline. The use of CT for adult emergency department visits for diagnosis of appendicitis has increased significantly, with sensitivities ranging from 85.7% to 100%, and specificities ranging from 94.8% to 100% 1.
From the Research
Normal Appendix Diameter
- The mean appendiceal diameter in adults was found to be 6.6 mm (±1.7) in a study published in the American Journal of Surgery 2.
- Another study published in PloS one found the mean maximal diameter of the appendix to be 8.19 mm±1.6 (SD) (range, 4.2-12.8 mm) 3.
- A study in AJR. American journal of roentgenology found that the mean appendiceal diameter was 5.6 ± 1.4 and 5.7 ± 1.5 mm for two reviewers, with 34% and 39% of appendixes measuring larger than 6 mm in pediatric patients 4.
Factors Affecting Appendix Diameter
- Patient age was found to be significantly associated with appendiceal diameter, with diameter increasing by 0.4 mm/y until 6-7 years of age, after which it remained stable 4.
- The quantity of pericecal fat and type of appendiceal content were multivariate predictors of diameter 4.
- Lymphoid stimulation was a multivariate predictor of diameter for one reviewer 4.
Diagnosis of Appendicitis
- A study published in the American Journal of Surgery found that while appendiceal diameter was larger in patients with appendicitis, >20% of patients without appendicitis had an appendiceal diameter >7 mm, suggesting that diameter alone should not be relied upon to diagnose appendicitis 2.
- Another study published in AJR. American journal of roentgenology found that a new CT criterion based on the maximum depth of the intraluminal appendiceal fluid greater than 2.6 mm is useful for differentiating appendicitis without periappendiceal inflammation from a normal appendix with a diameter greater than 6 mm 5.