ASCA IgG (Anti-Saccharomyces cerevisiae Antibody, IgG)

A positive (elevated) ASCA IgG means IgG antibodies to Saccharomyces cerevisiae were detected above the lab's threshold. This pattern is seen far more often in Crohn's disease than in ulcerative colitis, so a positive result raises the likelihood of Crohn's — particularly when ASCA IgA is also positive and pANCA is negative.

A positive ASCA IgG does not by itself confirm Crohn's disease: some people with ulcerative colitis, celiac disease, or no IBD at all can test positive. In people already diagnosed with Crohn's, ASCA positivity is associated with earlier disease onset, small-bowel (ileal) involvement, and a more complicated, aggressive course (strictures, fistulae, or need for surgery). Higher levels and positivity across more markers strengthen that association. Interpretation should always combine this result with clinical findings, endoscopy, and the rest of the IBD panel.

A negative (non-reactive) ASCA IgG means IgG antibodies to Saccharomyces cerevisiae were not detected above the lab's threshold. This does not rule out Crohn's disease — a meaningful share of people with Crohn's are ASCA-negative, and some are positive only for the IgA class. It's worth checking the ASCA IgA result before drawing conclusions.

A negative ASCA IgG is more consistent with the absence of Crohn's-type serology, and in the right clinical picture (especially with a positive pANCA) can support ulcerative colitis over Crohn's. As with a positive, a negative is interpreted alongside the full panel and clinical assessment, not on its own.

Anti-Saccharomyces cerevisiae antibody (ASCA) IgG is an immune protein directed against a sugar in the cell wall of common yeast — not against your own tissue. It appears far more often in people with Crohn's disease than in ulcerative colitis, so it's used alongside pANCA and other markers to support the differential diagnosis of inflammatory bowel disease. On its own it can't diagnose or rule out Crohn's, but a positive result — especially paired with a positive ASCA IgA — raises the likelihood of Crohn's and, in people already diagnosed, is associated with earlier onset and a more complicated disease course.

Note on the equivocal range: results that fall between the negative and positive cutoffs are reported as equivocal — neither clearly negative nor positive. An equivocal ASCA IgG usually calls for correlation with the other markers and, sometimes, repeat testing.