What this test measures
ASCA IgG detects IgG-class antibodies against mannan, the cell-wall sugar of baker's yeast Saccharomyces cerevisiae. IgG antibodies reflect a more established immune response than IgA, and ASCA IgG is positive in a slightly higher proportion of Crohn disease patients than ASCA IgA alone.
In most labs ASCA IgA and IgG are ordered together. The combination — both positive — is more specific for Crohn disease (about 95%) than either antibody alone (each about 90%). Together with ANCA testing, this forms the standard IBD serology panel.
Why it matters
IBD is becoming more common in urban Indian populations, with Crohn disease historically less common than ulcerative colitis but rising fast. When endoscopy and imaging cannot clearly distinguish the two — for example, when colonic-only Crohn looks like ulcerative colitis, or "indeterminate colitis" patients — ASCA serology helps. A positive ASCA IgG (especially with positive IgA and negative ANCA) supports a Crohn disease diagnosis and may predict more aggressive disease behaviour with earlier surgical needs.
It is important to know what this test does not do: it does not screen for IBD in symptomatic patients (endoscopy + biopsy remains the standard) and it does not track disease activity (calprotectin and endoscopy do). About 30–40% of confirmed Crohn patients are ASCA-negative, so a negative test does not rule out the disease.
How to prepare
No fasting required. Continue all medications including IBD therapies — these do not interfere with the antibody assay.
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 IgG (U/mL)[1][2] | Negative: < 20 U/mL (assay-dependent) | Negative does not rule out Crohn disease — about 40% of Crohn patients are ASCA negative. | Positive ASCA IgG supports Crohn disease, especially with positive ASCA IgA and negative ANCA. Combined IgA + IgG positivity = high specificity. Higher titres associate with ileal involvement and stricturing disease. |
IBD serology patterns
| ASCA IgA | ASCA IgG | ANCA | Suggests |
|---|---|---|---|
| + | + | − | Crohn disease (high specificity) |
| + | − | − | Crohn disease (moderate confidence) |
| − | + | − | Crohn disease (moderate confidence) |
| − | − | + perinuclear | Ulcerative colitis |
| + | + | + | IBD-unclassified / indeterminate |
| − | − | − | IBD not excluded — many patients seronegative |
Frequently asked questions
Why are ASCA IgA and IgG both tested?
Combined positivity is much more specific for Crohn disease than either alone. Many labs report both on a single panel.
Do I need to avoid yeast-containing foods before the test?
No. Antibody levels reflect immune memory and do not change with diet.
My ASCA IgG is positive but ASCA IgA is negative — what does that mean?
A single positive isotype is suggestive but less specific than dual positivity. Your doctor will interpret this alongside endoscopy, imaging, ANCA, and CRP/calprotectin.
Can ASCA replace colonoscopy?
No. Colonoscopy with biopsy remains the gold standard to diagnose, classify, and monitor IBD. ASCA only adds to the picture.
Will my ASCA level drop with treatment?
No. ASCA titres are usually stable over time and do not track disease activity. Doctors use faecal calprotectin and endoscopy for monitoring.
Do healthy people have positive ASCA?
A small percentage (~5%) of healthy people, and some first-degree relatives of Crohn patients, have positive ASCA without disease. Clinical context matters.
How quickly is the report ready?
Typically 2–3 days for ASCA panel.
Related Autoimmune / Rheumatology tests
Tests commonly ordered alongside ANTI SACCHAROMYCES CEREVISIAE IGG 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
- AGA Clinical Practice Guidelines — IBD · accessed 2026-05-30T00:00:00.000Z
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