ASCA IgG (Anti-Saccharomyces cerevisiae Antibody, IgG)
Other names: ASCA IgG, ASCA (IgG), Anti-Saccharomyces cerevisiae Antibody IgG, Anti-Saccharomyces cerevisiae Antibodies IgG, Saccharomyces cerevisiae IgG, Saccharomyces cerevisiae Antibody IgG, S. cerevisiae IgG, Anti-yeast antibody IgG, ASCA immunoglobulin G
Optimal Result:
0 - 20 Units.
What ASCA IgG measures
ASCA IgG measures IgG-class antibodies against mannan, a carbohydrate in the cell wall of Saccharomyces cerevisiae — the yeast used in baking and brewing. Despite appearing in an autoimmune-type condition, ASCA is not an autoantibody: it targets a yeast (dietary and gut) antigen, not the body's own tissue. Its presence is thought to reflect a loss of immune tolerance to microbial antigens at the gut barrier, which is why it's informative in inflammatory bowel disease.
ASCA is measured in two antibody classes, IgG and IgA. They're reported separately because testing both catches more cases: about 80% of people with Crohn's disease are positive for ASCA IgG, ASCA IgA, or both. This page covers the IgG class; the companion measurement is ASCA IgA.
Why it's tested
ASCA IgG is ordered as part of an inflammatory bowel disease (IBD) work-up, usually together with atypical pANCA. The two markers point in opposite directions:
- ASCA-positive / pANCA-negative leans toward Crohn's disease
- ASCA-negative / pANCA-positive leans toward ulcerative colitis
Used together, this combination can reach a specificity as high as ~99% for separating Crohn's from ulcerative colitis. It's a less invasive adjunct to the diagnostic gold standard (endoscopy with biopsy), not a replacement for it.
What the numbers mean
ASCA IgG is reported against a threshold, with results classed as negative, equivocal, or positive (see the reference range and tiers). Roughly:
- Crohn's disease: frequently positive
- Ulcerative colitis: positive in fewer than ~15% of cases (IgG)
- Healthy people: positive in fewer than ~5% (and essentially never positive for both IgG and IgA at once)
Because a small share of healthy people and some people with other gut conditions (including celiac disease) can test positive, ASCA IgG is interpreted in context — never as a standalone diagnosis.
What to do next
A positive or negative ASCA IgG is one input, not a verdict. Results should be reviewed by your clinician alongside symptoms, other IBD markers, and endoscopic or imaging findings. If you were tested because of ongoing digestive symptoms, bring the full panel — ASCA IgA, pANCA, and any glycan markers — to that conversation rather than a single value.
Lab Results Explained and Tracked
What does it mean if your ASCA IgG result is too high?
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.
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What does it mean if your ASCA IgG result is too low?
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.
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