The Performance of the APPY1 Test in the Evaluation of Possible Acute Appendicitis

Inclusion Criteria: * 2 to 20 years old (up until the 21st birthday), * Right lower quadrant or poorly localized abdominal pain, or tenderness that includes the right lower quadrant, and/or other features including, but not limited to, nausea, vomiting, and/or anorexia, requiring acute appendicitis to be added to the differential, * Onset of abdominal pain within 72 hours or less, * Patients of either gender may participate and, * Patient or accompanying parent or guardian is able to provide informed consent. Exclusion Criteria: * History includes prior appendectomy, * Subjects whose presentation history and physical examination place them at such high risk of acute appendicitis that it would be clinically reasonable to proceed with exploratory laparotomy and/or appendectomy without advanced imaging, * Treatment with any immunosuppressive medication or chemotherapeutic agents within 28 days, or systemic steroids (oral or intravenous) within 14 days, * History of end-stage, metastatic cancer or an active immune disorder, * History includes abdominal trauma or invasive abdominal procedures/surgery within the previous two weeks, * Patients who have received diagnostic imaging (CT, MRI, or US) for abdominal pain in the previous two weeks, * Patients with report of abdominal pain greater than 72 hours, * History of active bleeding disorder, which may complicate phlebotomy or placement of peripheral IV catheter, * Participation in a research study within the previous 30 days, * Prisoners of an adult or juvenile detention center, and * Subject, parent or guardian unable to provide informed consent.