This study will consist of a brief interaction with the subject or parent or guardian to obtain consent, collect a blood sample, medical history, and physical examination of those who meet the inclusion criteria. The blood sample will be tested for the WBC value, as well as processed into plasma in order to compute the APPY1 Test result, which is based on a combination of the WBC value and the plasma MRP 8/14 and CRP concentrations received from the APPY1 Test cassette. There will be a 2-week (14 days +/- 3) follow-up telephone call for those subjects that were discharged from the ED without an appendectomy or diagnosis of acute appendicitis. Use of the APPY1 Test to help identify low risk patients could significantly reduce the use of CT scans in diagnosis of AA, thereby reducing the exposure to ionizing radiation in children, adolescents, and young adults.

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.

Children, adolescents and young adults 2 to 20 years old.