Gastrointestinal Health Marker Guide

Secretory IgA

What this marker measures

Secretory IgA is a key immune antibody measured in stool that reflects mucosal immune activity in the gut. It helps protect the gut lining by binding microbes and toxins, limiting pathogen attachment, and supporting host–microbiome balance^1–3. Interpret alongside symptoms and other inflammation markers.

This assay is issued under the European IVDR framework (the assay is classified IVDR Class C) with ARTG listing for use in Australian markets
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Assay indications

Adapted from the instructions for use

Secretory IgA playsa major rolein preventing adherence of microorganisms to mucosal sites, in activation of the alternative complement pathway and in activating inflammatory reactions.

- Proof of an imbalanced immunological barrier on the intestinal mucosa

- Autoimmune disease

Clinical associations*

Consider this marker when your patient presents with:

Mucosal immune activation

High faecal sIgA may indicate increased mucosal immune activity in response to microbial, dietary, or infectious triggers. Interpret alongside symptoms and inflammatory markers such as calprotectin or lactoferrin.

IBS-D presentation

Faecal sIgA may be elevated in some patients with diarrhoea-predominant IBS, suggesting mucosal immune activation may be relevant.

Autoimmune context

Faecal IgA alterations have been reported in systemic lupus erythematosus, but evidence is exploratory; interpret in the context of broader autoimmune and gastrointestinal findings.

*In addition to the assay’s intended use, all clinical associations have been reviewed by the Microba science team to ensure clinical validity supported by Microba’s cited literature.

Interpreting the result

LOW

Below 500 ug/ml.

May suggest reduced mucosal immune activity or reduced secretory immune response at the gut surface. Clinical significance is context-dependent; interpret alongside symptoms, infection history, medications, immune status, and other gut markers.

WITHIN RANGE

500--2000 ug/ml

This suggests mucosal immune activity is not abnormal based on this marker alone

HIGH

Above 2000ug/ml

May indicate increased mucosal immune activation in response to microbial, infectious, dietary, or inflammatory triggers. Interpret alongside symptoms, stool pathogen testing where indicated, and calprotectin or lactoferrin.

Treatment guidance

Approach depends on whether levels are low or high.

IF LOW —Supplementation

Bifidobacterium animalis ssp. lactis Bl-04 may increase faecal secretory IgA⁴ Out-of-range result: referral to a medical specialist is warranted if cause unknown.
GRADE C

GOS (galacto-oligosaccharides) supplementation may increase faecal sIgA⁵GRADE D

Bifidobacterium animalis ssp. lactis BB-12 may increase faecal secretory IgA⁶GRADE D

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IF HIGH — Consider interventions

Investigate possible drivers of mucosal immune activation, including GI symptoms, infection risk, inflammatory markers, and relevant stool pathogen testing where indicated

Tips for discussing out-of-range results

Secretory IgA is part of your gut's immune defence at the surface lining. A result outside the expected range, whether low or high, can mean this immune barrier is a little out of balance, which is something we can look into together alongside your symptoms and history.

Evidence grading for patient management insights

The letter grades shown next to each patient management insight show the quality of the research behind it. Every insight provided has been through a rigorous review of the scientific literature and graded using the NHMRC Levels of Evidence, so you can see exactly how strong the evidence is before applying it in practice.

Secretory IgA- Reference sourcesSource references for all clinical associations, interpretation definitions, and patient management insights on this card.

1. León, E. D. & Francino, M. P. Roles of Secretory Immunoglobulin A in Host-Microbiota Interactions in the Gut Ecosystem. Front. Microbiol. 13, (2022).
2. Huus, K. E., Petersen, C. & Finlay, B. B. Diversity and dynamism of IgA−microbiota interactions. Nat Rev Immunol 21, 514–525 (2021).
3. Lisicka, W. et al. Immunoglobulin A controls intestinal virus colonization to preserve immune homeostasis. Cell Host & Microbe 33, 498-511.e10 (2025).
4. Zhang, H., Miao, J., Su, M., Liu, B. Y. & Liu, Z. Effect of fermented milk on upper respiratory tract infection in adults who lived in the haze area of Northern China: a randomized clinical trial. Pharmaceutical Biology 59, 645–650 (2021).
5. Vulevic, J., Juric, A., Tzortzis, G. & Gibson, G. R. A Mixture of trans-Galactooligosaccharides Reduces Markers of Metabolic Syndrome and Modulates the Fecal Microbiota and Immune Function of Overweight Adults. The Journal of Nutrition 143, 324–331 (2013).
6. Kabeerdoss, J. et al. Effect of yoghurt containing Bifidobacterium lactis Bb12®on faecal excretion of secretory immunoglobulin A and human beta-defensin 2 in healthy adult volunteers. Nutr J 10, 138 (2011).