What this test measures
ASCA (Anti-Saccharomyces cerevisiae antibodies) detects IgA-class antibodies against mannan, a sugar component of the cell wall of baker's yeast (Saccharomyces cerevisiae). The link to Crohn disease was discovered incidentally — Crohn patients tend to produce antibodies against this commensal yeast as part of a broader breakdown in intestinal immune tolerance.
The test is part of an IBD serology panel that typically also includes ASCA IgG and ANCA (anti-neutrophil cytoplasmic antibodies). Combined IgA + IgG positivity is more specific for Crohn disease than either alone.
Why it matters
Inflammatory bowel disease (IBD) is rising in India, particularly in urban populations. Ulcerative colitis (UC) and Crohn disease present with overlapping symptoms — diarrhoea, abdominal pain, weight loss, blood in stool — but the treatment, prognosis, and surgical options differ. Distinguishing them sometimes requires serology when endoscopy and imaging are inconclusive, especially in "IBD-unclassified" cases.
ASCA positivity (especially IgA + IgG together) supports a Crohn disease diagnosis. Conversely, ANCA positivity supports ulcerative colitis. ASCA-positive Crohn patients also tend to have more aggressive disease — ileal involvement, stricturing or penetrating behaviour, earlier need for surgery — making the test useful for prognosis. ASCA is not a screening test for IBD; it is used in established IBD to refine the diagnosis.
How to prepare
No fasting required. Continue all medications including any IBD treatments (mesalamine, steroids, biologics) — these do not interfere with the antibody test.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| ASCA IgA (U/mL)[1][2] | Negative: < 20 U/mL (assay-dependent) | Negative does not rule out Crohn disease. Sensitivity of ASCA IgA alone is about 30–40% for Crohn disease. | Positive ASCA IgA supports Crohn disease in the right clinical setting, particularly when ASCA IgG is also positive and ANCA is negative. Higher titres associate with more aggressive disease. |
IBD serology — interpreting ASCA + ANCA together
| Pattern | Suggests |
|---|---|
| ASCA+ / ANCA− | Crohn disease (sensitivity ~55%, specificity ~90%) |
| ASCA− / ANCA+ (perinuclear) | Ulcerative colitis (sensitivity ~60%, specificity ~90%) |
| ASCA+ / ANCA+ | Indeterminate — possible IBD-unclassified |
| ASCA− / ANCA− | IBD not excluded; many patients are seronegative |
Frequently asked questions
Do I need fasting for this test?
No. ASCA IgA can be tested any time of day.
Can ASCA diagnose Crohn disease on its own?
No. ASCA is supportive but not diagnostic. Diagnosis still requires endoscopy with biopsy, imaging (MR or CT enterography), and clinical correlation.
Why test both IgA and IgG?
Combined IgA + IgG positivity gives much higher specificity for Crohn disease than either alone. Many labs offer both as a single panel.
Does eating baker's yeast cause this antibody?
Everyone is exposed to baker's yeast in bread and beer, but only people with disrupted intestinal immunity (most often Crohn disease) make persistent antibodies. Dietary yeast does not need to be avoided.
Will treatment lower my ASCA level?
ASCA titres tend to remain stable regardless of treatment and disease activity. They are not used to monitor IBD response — faecal calprotectin and endoscopy are.
How long does the report take?
Most labs report ASCA within 2–3 days.
Related Autoimmune / Rheumatology tests
Tests commonly ordered alongside ANTI SACCHAROMYCES CEREVISIAE IGA ANTIBODIES, or that help interpret an unexpected result.
Sources & references
- NCBI StatPearls — Crohn Disease · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — Crohn Disease · accessed 2026-05-30T00:00:00.000Z
- Mayo Clinic Labs — ASCA Antibodies · accessed 2026-05-30T00:00:00.000Z
- American Gastroenterological Association — IBD Guidelines · accessed 2026-05-30T00:00:00.000Z
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