Compositions and methods for determining the likelihood of appendicitis

A method for determining the relative abundance of microorganisms corresponding to one or more operational taxonomic units (OTUs) in a test biological sample in a subject, comprising:

• (a) detecting the presence of microorganisms corresponding to one or more operational taxonomic units (OTUs) in a test biological sample obtained from a subject suspected of possibly having appendicitis, wherein at least one OTU is selected from the group consisting of Bulleidia and Dialister;
• (b) determining the relative abundance of the microorganisms detected in step (a);
• and
• (c) comparing the relative abundance of the microorganisms in each of the one or more OTUs to a corresponding reference value assigned to each of the one or more OTUs that is representative of a subject that does not have appendicitis,
• wherein the test biological sample is a rectal swab sample,
• wherein step (a) comprises amplifying a marker gene from microorganisms in the one or more OTUs, and
• wherein a significant difference in the relative abundance of Bulleidia and/or Dialister is detected in the test biological sample as compared to the corresponding reference value or values which significant difference indicates a high risk of appendicitis in the subject.