What ASCA IgA measures
ASCA IgA measures IgA-class antibodies against mannan, a carbohydrate in the cell wall of Saccharomyces cerevisiae — the yeast used in baking and brewing. Despite showing up 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, IgA and IgG. They're reported separately because testing both catches more cases: about 80% of people with Crohn's disease are positive for ASCA IgA, ASCA IgG, or both. This page covers the IgA class; the companion measurement is ASCA IgG.
Why it's tested
ASCA IgA 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 IgA is reported against a threshold, with results classed as negative or positive (see the reference range and tiers). Roughly:
- Crohn's disease: frequently positive
- Ulcerative colitis: positive in fewer than ~2% of cases (IgA)
- Healthy people: positive in fewer than ~5% (and essentially never positive for both IgA and IgG at once)
Because a small share of healthy people and some people with other gut conditions (including celiac disease) can test positive, ASCA IgA is interpreted in context — never as a standalone diagnosis.
What to do next
A positive or negative ASCA IgA 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 IgG, pANCA, and any glycan markers — to that conversation rather than a single value.
ASCA IgA
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